2.1 is definitely the fastest available, but as far as stability IMO there are no differences between 2.1 and 2.5. Most chef's include the thing's you would have to tweak on the stock rom's..EX. no push internet, disabling annoying notification's and improving battery life. I personally like 2.5 and 6.5, they are both touch friendly. The newer 6.5 rom's have extra tab's like twitter, footprint, and facebook. My advice is to try as many as possible to find what you really like and enjoy, then pass along your advice to the community. My personal choice is MR.X 2.5, CFC compressed makes them super fast! Go to his thread for more info
http://forum.ppcgeeks.com/showthread.php?t=110661.