I don't know anything about Mike Portnoy(I plan on doing research and listening to his work after this post), but Danny Carey can do what Neil Peart can with a third of the drum set...All that spinning around Neil does on his 360 degree drum set is quite impressive, but Danny is better.
BTW, I put Peart at #2.
I'm with Roadie here... when Mr. Carey has done it as well for as LONG as Neil Peart, then we can talk about Greatest of All Time... until then... we're talking about Greatest Right Now. And that's still open to debate.
Oddly enough , the Professor might argue that someone like Buddy Rich is deserving of consideration as well. No question though that all of the folks above are clearly in pretty good company at the top of the heap.