This is the list I would play if I had a Legacy event tomorrow. It's well-balanced, is difficult to hate out, and has strong game against every popular deck other than Dredge, Belcher, and Mono-Red Burn.
Deathrite Shaman and Tarmogoyf are two of the most universal cards in Legacy right now. They outclass all similar creatures at their respective spots on the mana curve. Four copies of each in the maindeck goes without saying. However, certainly consider trimming them during sideboarding (Tarmogoyf more so than Deathrite), and remember that they make you somewhat reliant on your graveyard.
Dark Confidant: When Jarvis Yu and I began working on our BUG Midrange decklist two months ago, we couldn't figure out why Legacy players seemed to have forgotten about old Bob. Dark Confidant is one of the best, most influential cards in Modern, and it's even better in Legacy! Curves are lower, life matters less, and card advantage is tougher to come by. I'm tempted to call Confidant the best card in Legacy Jund in the sense that it's the card that most easily and directly leads to a win. Use Thoughtseize to stick him early and then trade one for one as often as possible and watch your opponents crumble.
Bloodbraid Elf is the perfect curve topper but not much beyond that. Nearly every deck in Legacy either kills you in the first three turns or Wastelands you out of the game, so the last thing you want to look at is multiple four-drops in your opening hand. However, the mere presence of Bloodbraid in your deck dramatically reduces the value of opposing planeswalkers, most specifically Jace, the Mind Sculptor. Bloodbraid is never bad, and I've added the fourth copy to the sideboard to come in in grindy matches like the mirror or whenever you have too many weak cards. A good example would be the Miracles matchup, where it's helpful to reduce your reliance on the graveyard to hedge your bets against Rest in Peace and Relic of Progenitus.
Bloodbraid becomes better after sideboarding for two reasons. First, your deck becomes more streamlined (better) for the matchup in question, so you're more likely to cascade into something good; second, both players are more likely to load up on removal, so there's a higher chance of the game dragging on and powerful, expensive spells determining the winner.
Scavenging Ooze and Tarmogoyf compete as the best green two-drops in Legacy. In some decks, the Ooze wins out, but Jund is not one of them. Jund is all about efficiency and typically uses all of its mana on turns 2 through turns 4 or 5. The fact that Goyf can attack for four or more right away makes it brutal enough to put your opponent on the back foot immediately. Ooze has to sit there as a 2/2 for a few turns, vulnerable to Lightning Bolt, before it can begin to really shine. If Jund needed more two-drop creatures, I could see one Ooze being played in addition to the four Tarmogoyfs, but in reality, the last thing the deck needs is another two-drop that doesn't have a large, immediate impact on the game.
Grim Lavamancer: I played two Grim Lavamancer at Grand Prix Denver; in fact, they were a large reason why I was drawn to the deck in the first place. However, they ended up underperforming. I anticipated them being good against RUG Delver. While they were okay in game 1, Pyroblast and Lightning Bolt give you plenty of answers to Delver of Secrets, and Lavamancer is unimpressive against Tarmogoyf and Nimble Mongoose. They can often make your mana decisions awkward, as the basic Mountain doesn't cast many of the key spells, and it's inconvenient to fetch out a dual land on turn 1 against many decks.
Finally, Lavamancer is one extra card that depends on the graveyard, and I view the ability to operate under a Rest in Peace as a good tiebreaker between two similar cards. Lavamancer is great against Merfolk, but that deck is on the downswing and is already a great matchup anyway. It was instrumental in Pat Cox's quarterfinal win over Elves, but even so, I feel fine cutting them or at least moving them to the sideboard.
Sylvan Library is like a Dark Confidant that cannot be killed by creature removal. It's downright insane against any deck that doesn't attack your life total, as it represents an Ancestral Recall in addition to a free-to-activate Sensei's Divining Top for the rest of the game. Even in aggro matchups, it provides insurance against mana screw and flood and can easily swing a game that goes long. Library is one of the best cards against Miracles and Stoneblade, and I would definitely consider a second copy in the sideboard.
Life from the Loam was in my sideboard in Denver, and it was the single card I brought in most frequently. In any "fair" matchup, it allows you to utterly dominate one of the most important aspects of the game. If you draw a Wasteland and a Life from the Loam, you dramatically reduce your opponent's ability to function and take away any chance of them coming back from behind. Even with just fetchlands, it means your opponent can never mana screw you, means you'll hit four mana when you want to, and means you'll have infinite fodder to discard to your Liliana of the Veil.
Liliana of the Veil: I'm not sure that "engine card" is the best way to categorize Liliana because she functions more to tighten things down than to fuel unfair shenanigans. However, it's Liliana that really allows you to lock things up, and she's equally good against aggro, control, and combo. On defense, her Edict ability serves as removal that can answer shroud, hexproof, and protection creatures. When you're winning, she deprives the opponent of resources and takes away any chance of a comeback.
Removal and Discard
I won't cover each of these cards individually since their function should be apparent. However, it's worth noting that Jund has a slight deficiency of one-mana plays and an overabundance of two-drops. For this reason, there are only three copies each of Hymn to Tourach and Abrupt Decay despite them being among the best cards to see one copy of in a game. Hymn in particular can be clunky to draw in multiples.
Thoughtseize is the best spot discard spell in any matchup other than Mono-Red Burn. Beyond that, Duress is best for the sideboard since the extra discard comes in mostly against combo decks and Miracles anyway. If I was to add a fifth one to the maindeck, though, it would be Inquisition of Kozilek.
Maelstrom Pulse is a little unexciting but provides a valuable out to Lingering Souls, which is the single best card against Jund. With so many two-drops in the deck, it's common to play one on turn 3 and waste a mana anyway, so the mana inefficiency of Pulse is often irrelevant. It's generally great in the Stoneblade matchup since it kills Lingering Souls, Jace, and Batterskull depending on the situation.
There's a certain pattern that I've noticed. About once per year, I enter a Legacy tournament without Pyroblast in my sideboard. Equally often, I swear a blood oath that I'll never do it again. Pyroblast is better than Force of Will, better than Thoughtseize, better than Counterspell, Spell Pierce, Daze, Flusterstorm, and any other disruptive card you can think of. Pyroblast is the best sideboard card in Legacy, and it's the best reason to play Jund over BUG or Esper Stoneblade.
As an aside, Jund should play Pyroblast over Red Elemental Blast for the times that you cascade into it with no blue permanents in play. With Pyroblast, you have the option to target a nonblue permanent; it won't have any effect, but at least it'll go to the graveyard for the purposes of Deathrite Shaman and Tarmogoyf (and Grim Lavamancer).
Matchups and Sideboarding
BUG and Jund have a huge number of cards in common, so the matchup is inevitably going to be close. If one player has a much better draw than the other, they will win. However, the red cards are better than the blue cards in the pseudo-mirror. In particular, having a one-mana answer to opposing Deathrite Shamans and Dark Confidants puts you in a great position to pull ahead in the early game. Bloodbraid Elf is more reliable than Jace and happens to match up great against the planeswalker in a direct fight. Brainstorm is a great topdeck on turn 7, but finding the right plays in the early turns is so important in this matchup that the BUG player will be tempted to cycle it early if it's in their opening hand—really nothing to worry about.
As a general rule, it's okay to trim one Hymn to Tourach against any non-combo deck when you're on the draw. I would try to keep them all on the play.
A tough, grindy matchup. Plan ahead to make the most of Liliana battles.
You'll lose to their best draws, but on the whole this is not a matchup to be overly scared of. The post-board games will favor Jund, especially on the play.
This is all about attacking from multiple angles. You need discard spells to ensure that you survive the early turns. However, it'll be Liliana of the Veil, Chains of Mephistopheles, Nihil Spellbomb, Wasteland, and Pyroblast that really put the game out of reach for the Storm player; you'll need to continue to choke their resources throughout the game so they can't rebuild their hand.
Similar to BUG, this will be a close matchup where if one player draws much better they'll win. All things equal, though, Jund's cards are more efficient and put pressure on faster, so you'll have the advantage. In game 1, you want to be ahead on the board when Souls start Lingering.
With Lingering Souls, Stoneforge Mystic, and Snapcaster Mage all providing guaranteed value, you'll have to rely relatively less on your spot removal in this matchup. While you can't exactly let Stoneforge do its thing, a nice alternative to spending removal on the Mystic is to Thoughtseize or Ancient Grudge the equipment after it's been searched up. Post-board, you have access to all the right answers, and it'll only take a Dark Confidant or a Bloodbraid Elf to pull you ahead once you completely shut them down.
Bonus: Standard Jund Decklist
Here's the deck I've been playing on Magic Online lately. As usual, Jund can be tuned to beat whatever you really want to beat. Online, these days, the most important matchups seem to be Bant Hexproof and Red Aggro.
The removal suite is tough to get exactly right. I feel two Abrupt Decay is the right number to be able to answer Runechanter's Pike and to kill opposing Detention Spheres. At times, I've wanted access to Dreadbore, Mizzium Mortars, and Sever the Bloodline, but on the whole I feel Ultimate Price and Bonfire of the Damned get the nod. Searing Spear doesn't belong in this type of deck; it's rare that you'll be able to deal the opponent seventeen damage but not twenty. Given that, Ultimate Price is just better for killing creatures.
Those are my takes on Jund in two of Magic's more popular and exciting formats. Give these decklists a try; I'm confident you won't be disappointed!