For me and in no order except maybe for the first one:
- Bill Cosby. Him doing what he did sucks but doesn't change how funny his material was. I think "Himself" was the best comedian show ever, but his older stuff is also classic...he also deserves high praise for being able to be funny without being in the sewer or even swearing.
- George Carlin. The absolute master of language and I loved his observations of it. I thought his "angry man" bit towards the end of his life however was weak.
- Don Rickles. He wouldn't get to first base in today's political-correctness-gone-viral world. Thank God he came along sooner and was one of the best ever. Also the best comedian to have as a talk show guest; his stuff with Johnny Carson (but also elsewhere) was so great.
- Leo Gallagher. Easily wins my "most underrated" award. Some dismiss him as just a "prop comedian" which is a joke, he was so much more (but even his prop stuff was great).
- Dana Carvey. My favorite "absurdist." I esp love his musical bits.
- Robin Williams. Self-explanatory.
There are many others of course, but if I had to narrow it down to roughly a handful, to me these are the most memorable.