Grissom is (was) the perfect balance of knowledge, wisdom and "gallows humor" (and maybe "cynicism") whose complex character was (IMO) the linchpin of CSI. He was the most-qualified/best CSI there, and could have rose to the top, but he wouldn't (as Sara Sidle told Conrad Eckley) "kiss-ass."
Mac... a real bastion of integrity. Less egghead-ish than Grissom, but a great lead character in his own right.
Horatio... IMO is like a modern-day "Steve McGarrett (of "Hawaii Five-O" fame)--except with those trademark glasses, and trademark way of "taking them off." Like McGarrett, Horatio's hare never seems to move (except maybe "ever-so-slightly" in the Miami breezes).