Louis CK - In the last ten years, I feel like the only shows that have regularly delivered great big belly laughs are Louie and Chappelle's show. I love his honesty. He's not afraid to let people know that he's an !@#$. Where Carlin focused on society, Louie CK will often use himself as an example of how !@#$ed up society is.
Mitch Hedberg - Any comedian relies on timing, and IMO Mitch really started a whole generation of comics who are currently playing with using timing differently, and thinking about jokes in a different way.
George Carlin - I got my first George Carlin cassette when I was about 10 years old, and wanted to be a comedian for about 5 years. That same album still cracks me up, although it may be partially nostalgia.
Honorable Mention - I don't think he actually does stand up, but one guy who I know will always crack me up is H Jon Benjamin. He's done a lot of voice work in animated shows, and anytime he's in something I know I'll enjoy it. Here's a clip from Home Movies. He was also on Dr. Katz.