It's very rare that, after a week of drafting a format, I have absolutely no clue what the best common could be, but that's the case for Masters 25.
There are so many different archetypes, some linear and others far from it, that cards range from very high power level in one version of a color combination to a mediocre card in another deck of the same color-combination. I see a world where Hordeling Outburst is the best common, but there are red decks where that card just isn't impressive. There are decks where all you want are land cyclers, and then others that have little interest.
The safest powerful common is probably Murder, but if you're up against a bunch of Baloth Nulls or Cloudblazers, then you'd rather have that Counterspell. And with all the fixing, maybe it's better to start on something splashable and efficient like Disfigure, or maybe even Cultivate to help facilitate those splashes.
I think I've made my point: this format is deep, and navigating the draft can be quite difficult. You really need to understand what the likely plan is for your deck early on in order to make decisions between cards like Hordeling Outburst and Kindle.
Pack 1, Pick 1
While I don't think Twisted Abomination is the pick, I wanted to make it known that I'm not really upset if I open a pack where I first-pick a land cycler. It's certainly not what I would prefer, given the power level of the set, but they play very well and increase spell-density and consistency of your deck. And that's certainly a valuable asset.
Treasure Keeper is a fine card, but it's not as good as it once was. Yes, it's a reasonably powerful card without color requirements, so it's a fine card to start a draft with. However, I keep finding myself in a situation where in this format where it can't particularly attack or block profitably. It's great against aggressive decks, for a four-drop at least. I'm probably undervaluing Treasure Keeper based on my experience so far, but something just doesn't feel right.
So this leaves two red cards, and they do drastically different things. Frenzied Goblin facilitates a hyper-aggressive deck that leans pretty heavily on red sources, while Pyroclasm is a control-oriented card that's splashable. I'm comfortable drafting both of those decks in this format, and currently I'm of the opinion that they are both equally viable. I think Pyroclasm has a higher likelihood of making my deck, as I could get cut out of the red aggressive deck, but it's unlikely that I'll get cut out of any control or midrange deck with the capability of splashing Pyroclasm. As this is a reasonable tie-breaker, I'll be starting on Pyroclasm.
Pack 1, Pick 6
The Picks So Far:
Okay, I'll be honest here: I don't think Brainstorm is even in this discussion. However, I figured there may be a reasonable number of you that are aware of how powerful this card is in Constructed and also don't have experience with the card in Limited. It isn't very good. Sure, with shuffle effects it can be reasonable, but you're rarely going to get enough efficient shuffle effects to make it worth taking. It just doesn't provide enough flexibility and selection to justify a slot in your deck. Trust me.
The real question is between Twisted Abomination and Cultivate. Cultivate is inherently a more powerful card, given that it's a two-for-one, ramps you, and fixes you. However, I think U/B is a better shell in this format than U/G, and Twisted Abomination doesn't commit you to a color in the same way that Cultivate does. Personally, I think this puts Abomination above Cultivate, but it's close and could go either way!