There are tons to use. You can check out MIUI for something different or any of the good Sense based ROMS like the ones from Virtuous. The good thing is you can always flash back to the ROM you liked assuming you did a nandroid backup prior to flashing a new ROM. I am using a ROM from Team Nocturnal that is based on CM7 but has some Sony elements to it, it also seems to run faster than plain ole CM7.
Sense 2.1 has the world phone capabilities and it's what was originally on the phone. Sense 3.0/3.5 have different features such as the lockscreen, and the whole sense layout is just different.. AOSP is senseless and ICS is Android 4.0+ (also senseless).. Experiment for yourself and find out!