After the last draft I did, I got one booster pack as a prize, and didn't have enough time to look through it last night, so will do so now.
Interestingly, there are no white cards in the pack, but the other colours are fairly well represented. The top 4 cards are Unburial Rites, Rage Thrower, Mirror-Mad Phantasm and Mayor of Avabruck // Howlpack Alpha.
I think, of the four, the first card I would discount would be the Rage Thrower. While I have not played with it yet, the effect does look powerful, although I don't know how much of a limiting factor the high cost is.
As much as I like the card, the next one I would discount is Unburial Rites. I probably overvalue the card anyway, and I do really like the effect, but I can acknowledge that the other two cards are better choices.
The pick then comes down to Mirror-Mad Phantasm and Mayor of Avabruck // Howlpack Alpha. The Phantasm would force you into building around it (to try and mill yourself), so I think the Mayor would be the best pick here - it is good, easier to splash, and can go in most decks (I didn't play it in my last one, but Dan suggested I probably should have done as it is a great card). The only other green cards in the pack are Naturalize and Caravan Vigil, so it is not as though the player you are passing to is going to also be going into green straight away (they may have seen you take the double sided card).
I think the best thing about this prize pack is I got another card to put in this Laboratory Maniac deck I want to throw together :-D Mwahaha