I've always loved Grissom and hated Horatio.
Grissom is a very cool, intellectual, socially awkward but good at heart character (Kinda like an anti-House) to whom I feel very close. Of course, he is also the teamleader of the team I like the most from the period of time I actually liked CSI (right now I really really dislike it, I think I'm tired of all "procedural" shows).
I hate Horatio for being ridiculous and keeping to judge and accuse everybody around him of being terrible. Of course, he embodies the message of the whole CSI Miami series, wich is that everybody is an asshole for feeling happiness while there's people actually dying on the street (or at least that was what I got when I watched it. I stopped soon, so that may have changed with time, I don't know)