Greatest ROM ever: M I U I
It is so glorious.
Others that are worth mentioning however would be:
1. iBolt - this is a great ROM. At the time I installed it, iBolt was the closest thing to MIUI. No real issues with it other than not being able to get bluetoof to work. Also, I seemed to get poor battery life.
2. SkyRaider Zeus Preview - This one uses sense which I dont particularly care for, but I figured Id try it anyway. I actually installed this earlier today while bored before I stumbled upon the MIUI thread. Anyway, it was really snappy and responsive, lightening quick, and battery life seemed great for the few hours that I used it (heavy use messing around with settings and setting everything back up, hardly lost any battery)
3. Cubed Rom - This is the first ROM I ever used and it was a great choice! It had some issues at the time (around July I believe) but nothing too detrimental. Relatively quick, and had some nice, but pretty basic, features. I just googled it and it appears as of 10/5/11 it is no longer supported for whatever reason.
Pretty sure those are the only ROMs Ive used... Ill add more if I think of any, but hopefully that gives you a few things to try out

What I like to do is install a ROM, get everything redownloaded and installed, restore all my contacts, and set it up to my liking, then make a backup. This way, you can restore at any time to a perfect, clean version of whatever ROM you feel like running that day or week or whatever.