This is a tough one for me.
For me, Sagan is way up there. While his contribution to science is not as great as some, it was his love of science and his ability to transfer that love of science to his audience that, to me, was his greatest contribution. If it wasn't for Sagan, I don't think I'd have the love of science that I do.
Einstein is also pretty high on my list. It was through his contributions that our understanding of the universe that we can see is as defined as it is.
Stephen Hawking is almost as high on my list as Einstein, but I will admit a little bias in the fact that I have gobs and gobs of respect for that man. My grandmother died of ALS, and he has lived with it most of his life. Not only that, but look at some of the brilliance he achieved, even with his handicap! Mad props, man...
I also think that the man who invented microwave ovens should be honored. Without him, I would not have survived my bachelor years...