It also depends on what you're looking for. For example, I run Pete's ROM because I like its speed and battery life, and I don't really have a lot of interest in themes or need for the advanced features you'd find in something like Cyanogenmod. On the other hand, some people prefer Cyanogenmod just because of those neat features (360-degree screen rotation, N1-like launcher, apps2sd, etc). Still others like Adam's Smoked Glass or Chevy's ROMs for the wide range of customisation options available. And some people like the Ultimate Droid ROMs because Blackdroid basically copies other people's ROMs and adds his own theme.
It's all a matter of what you want from your phone and what you use it for. And some ROMs seem to just run better on some phones than others for whatever reason. Definitely recommend trying a bunch of them out and seeing what you like best.