Although I've only seen Guilfoyle in CSI- I think that the rest of the world would have to agree that it is by far his most memorable role.
My first impressions of his character in CSI were not altogether positive- an aggravated boss who liked control and didn't see eye to eye with my favourite Bugman, Grissom.
However, as the show progressed, and he took a slightly more background role- I've wanted him on screen more and more (ironic-huh?). Brass, along with Greg Sanders, are the chief scene-stealers out of all the CSI's. The best lines, quips and storylines are his to own- and Guilfoyle handles by portraying an almost comic character, but with razor-sharp intellect and zero-tolerance for the bad guys. More Brass-centered episodes have been both entertaining and interesting to watch, and I firmly believe that without Jim Brass, CSI would be a whole lot duller.