Well, it depends on what you're looking for. Kazmir probably has the highest ceiling in the minors but won't be up for a couple of years. Are you looking for overall potential (riskier) or who is ready to help soon and is good?
Thanks like apples and oranges, those guys are all so different. Upton probably has the most upside of any of them, but I'd be shocked if he was a regular before 2006. Young and Kotchman have monster power, but that doesn't necessarily translate into MLB success. Kazmir is a great pitcher, but won't have any effect this year. The only one that could play this year is Marte, but he's probably the least talented of the bunch.
That being said, if I HAD to rank them, it would go like this:
Umm, Peterson has said he thinks Kazmir is ready for the majors now even though he in on a 75 pitch count so far, dont be surprised to see him up in the majors later this year. But to answer the question.
Upton (a potential 5 cat SS)
Rios (a potential 5 cat outfielder)
Young (monster homerun potential)
Kazmir (will he start or will he close?)
Marte (possibly the next great hitting 3B)
Kotchman (huge offensive potential, but there is an injury concern with him)