Louis C.K. is pretty much at the top as far as stand-up for me. Other favorites include:
Mitch Hedberg
Dave Attell
Patton Oswalt
Russell Brand
Ricky Gervais
Demetri Martin
I enjoy many more, but there are also a lot of acts I haven't seen.
I'm pretty close to JP as far as talk show hosts. David Letterman, Conan O'Brien, Craig Kilborn (I really miss him), and Jon Stewart are all great. I like Jimmy Kimmel, but I never catch his show.
Comedy beyond those areas can make for a broad category. I would agree with Rowan Atkinson being a brilliant performer. Excellent physical comedy with Mr. Bean, and great biting wit as Blackadder.
Larry David has contributed quite a lot to the comedic landscape. I bow down to him.
I'm sure I could go on and on about people who make me laugh, but I'll stop there for now.
