I'd say Prince, since he is as close or closer then any of them and also has huge potential.
I'm sure someone will say Kotchman is the closest, but I don't see it to much like that. Though if your speaking about who could handle a job the best, it's prolly him, just not fantasy wise. First the Angels have a very solid team and solid bench, they don't really need him now. Also if they did, Kotchman won't give much unless he hits for avg, he doesn't have power that a 1B needs and he would be pretty far down in the line up.
Prince will start the year in Triple A from everything I hear. He is pretty much an Overbay injury or Overybay moving to the OF, away from getting some time. Prince also will hit for power, and is more polished then most his age at the plate. Besides that we know the Brewers suck and don't have a big reason not to give him a chance if he does well in the minors.
Stewart also has huge potential and I like him a lot. That being said he also has gotten a lot of hype lately, most of it being justified by his performance. I've heard the Rockies don't want to rush him to much and will start him in high A, though i'm sure he will be in AA this year later on. Anyways if you look at Prince's and Ian's stats both at the same age in Low A, there isn't that big of difference. The one that does stand out a lot is the K/BB ratio, Ian wasn't bad, but Prince was pretty amazing for his age.
For that reason and that Prince is one step away from the majors, makes me choose him over Stewart...though I do like both a lot.