1. CSI - the original and most definitely the best. Some of the storylines just can't be beaten, and who can argue with Grissom?
2. CSI:NY - I didn't used to like this as much as i do now, but it certainly has grown on me. It's a close second to Vegas. The first season was a little boring, but the second season was much better and i love it now.
3. CSI: Miami - I don't like this one that much. It annoys the hell out of me really, Horatio especially. The way that he always has to get close to the victims and look after them, and the whole sunglasses/hands on hips thing. That was amusing at first, but now its just weird. Miami always tries to make things too exciting, with explosions etc which it really doesn't need to. The others don't try that. Definitely the worst of the 3.