I think you have to look at ADPs of your specific league. ESPN has Zobrist at #90, Beckham at 122 and McKutchen at 115.
You have a 12 team league with 8 keepers, which means in a perfect world, the top 96 players will be taken. Granted you have the top pick which means you have the best chance of taking advantage of a stupid move. (ie. someone keeps a high upside player with zero experience in favor of a proven veteran). A keeper league obviously puts more value on younger high risk - high reward player, but a lot of people look at that and don't realize that you play next year too. In my keeper league I won my league with proven, unsexy veterans (Abreu, Helton, etc.) mixed with young up and comers (Votto, Ethier, etc.)
With the ADP in consideration, Zobrist (in ESPNs case) should be your choice if you're undecided. Here's why: Zobrist at #90 would make Zobrist most likely a first round pick in the redraft. McKutchen and Beckham would be a 2nd-3rd round pick, giving you the best likelyhood of picking them back up in the redraft.
As for Justin Upton: no. Don't do it. Upton is #24 in the rankings and has top 10-15 potential. Granted I placed less emphasis on the keeper aspect earlier, but this is your keeper opportunity. You have a young highly touted player who is expected to make this huge leap this year. Upton is the epitome of a keeper. Don't squander that in a trade.
Final note: Hamilton and Quentin should not and will not be MVP candidates. Temper expectations because Upton is your best OF and Hamilton and Quentin are decent #2 OFs in your league - #3s in a 10 team league.