I like Dark Motives (for PC) the best because the cases are challenging and the characters do the correct voices. 3 Demensions of Murder (once again, PC) had really good cases as well and I loved being able to work with Greg rather than him just being in the lab. It was also very fun to be able to examine the evidence yourself (in the lab) rather than Greg doing it.
I didn't feel that the characters really look like the characters in 3-DOM, but I really liked everything else. All in all, the three CSI computer games were really good. I'd get the very first game first, then work up to Dark Motives and then 3-DOM so A, you can learn the game, and B, you can get used to the graphics and the tools.
Happy playing!