Definitely Prime 2. Like some have said before better boss battles; the best in the trilogy overall because they were so rewarding once you..well got your new item every time you beat a boss. Plus the bosses were plenty challenging. The atmosphere in general was much better and the level design felt more artistically thoughtful and balanced. The soundtrack was better too. The light suit was awesome as hell.
The original Prime 2 didn't have motion controls everywhere so that's also a big plus; although even if one were to compare the trilogy Prime 2 with Prime 3 the aforementioned qualties.
Edit: Meant to put in "edges Prime 3 out quite easily" at the end of that sentence.
Finally..the introduction of the Screw Attack! Yeah Prime 3 had it too, but you have no idea how awesome it felt to finally use that ability for the first time IN 3D!
Lube Me Up
