Pack 1, Pick 1
First let's consider the question of which black card is better to start off a draft. Three mana for a 2/2 with situational evasion isn't a particularly great card. That being said, there's a solid number of one-toughness creatures that Plague Mare preys on. I think I may have overrated the card early on given that it destroyed me in both my first and second draft of this format. Now with more drafts under my belt, I think I would rather have Lich's Caress as a consistently powerful removal spell. That three life really does matter.
However, I think neither of the black cards compare well to Prodigious Growth. That card ends the game extremely quickly, and neither red nor green are going to be able to handle a threat that large. I think this is a very powerful rare, and I don't plan on passing it much.
Pack 1, Pick 4
The Picks So Far:
With a five-drop and a six-drop, is the ramp from Elvish Rejuvenator worth more than usual? Maybe, but I don't think it's worth prioritizing. The card is substantially worse than Druid of the Cowl, and while it's still playable, it doesn't compare well to the power level of either Meteor Golem or Star-Crowned Stag.
So, the question of this pick is between the expensive payoff card, or the reasonably efficient beatdown card. Prior to playing much of this format, I think I would have taken Star-Crowned Stag here. However, two observations after drafting about ten times are going to make me lean towards taking Meteor Golem. The first is that most colors, although specifically white, have an overly saturated four-drop slot. The second is simply that the Golem is so much more potent than I thought. I'm actively looking to cast it and always groaning when my opponents do. And green decks are well-positioned to be able to tap seven mana. So those combine to the logic that I think leads me to take Meteor Golem here.