Definitely Neil Peart. I saw him 2 summers ago and he's incredible, even as an old man. His drum kit has electonic drums that surpass Portnoy's level IMO. He's the best because he's so versatile, e.g. he can successfully drum in different genres like jazz. Peart uses many more experimental MIDI sounds than Portnoy with his electronic drums. His digital sound sample library is extensively amazing... and the unique sounds in conjunction with his crazy time sig's add a creativity to his solos that Portnoy cannot match.
Portnoy is good and may even surpass Peart in technical ability (young arms vs old) but I wasn't impressed with his performance on "Nightmare". In fact, I think the Rev (Avenged Sevenfold's dead drummer) was an all around better talent because he provided amazing backup vocals + very very good technical skill (almost on Portnoy's level). Their Nightmare CD sounded very hollow without Rev's vocals and Portnoy's drumming was mediocre IMO.
Portnoy on Peart in Modern Drummer interview from 10/2007:
My biggest and most obvious progressive drumming influence was Neil Peart. Before I heard Rush I was into The Who, Led Zeppelin, and The Beatles. Neil Peart’s drumming was the first time I’d ever heard a drummer playing a lot of odd time signatures, and his massive kit was immediately intriguing to me. The way he created drum parts that played such a lead role on the early Rush albums captured me. I couldn’t listen to Hemispheres, 2112, Permanent Waves, or Moving Pictures without immediately being drawn to the drumming. I was one of the kids obsessed with Rush and the drumming of Neil Peart in the late ’70s and early ’80s.
http://www.moderndrummer.com/updatefull/200001603/Mike%20Portnoy
As far as accomplishments:
Peart - writes Rush's lyrics, wrote 4 books, youngest drummer to make the Hall of Fame (beating Portnoy by 6 years), Modern Drummer awards: Best Rock Drummer X9 (no longer eligible for the award lol) and won Multi-PercussionistX4, Percussion IntrumentalsX1, Best All AroundX1, Best Promising New Drummer (early career), Best Instructional Video (2006 Anatomy of a Drum Solo), Best Drum Recording of the 80's (and 2007), and Best Recorded Performance X16 for every tour he's ever done.
DRUMS! magazine: He's either swept or flip-flopped runner up from 2007 - 2010 with Portnoy for Best Progressive and Mainstream Pop drummer and beaten Portnoy every year for Drummer of the Year.
With Rush, 24 gold and 14 platinum albums which are third only to the Beatles and R.Stones for most consecutive gold or platinum albums. They also have Canadian Hall of Fame status and estimated 40 million sold worldwide.
Portnoy - writes many of DT's lyrics, and has Modern Drummer awards:
"Best Up & Coming Talent" "Best Clinician"X2, "Best Educational Video/DVD"X2, "Best recorded performance"X6 and "Best Progressive Rock Drummer" X12 (for 12 consecutive years). Also Has 10 bootlegs out.
With Dream Theater, a couple of gold albums but no platinum. They have an estimated 8 million sold worldwide.
So all in all, I think ability wise: Peart by a little because he uses more electonic sampling, then Portnoy who is probably better technically off youth alone, and then everyone else. Overall resume: Peart by a mile with Rush and pretty even with respect to Modern Drummer awards where they are annually awarding Portnoy Best Progressive + Educational stuff (Clinician, Instuctional) and Peart Best Rock + Recorded Performance.
Some pics of their amazing kits:
Peart's Time Machine tour (current) kit:
http://traviswhitmore.com/post/820956388/themurph-neil-pearts-steampunk-inspired-drum
Portnoy's Progressive Nation tour kit:
http://www.metalsucks.net/wp-content/uploads/2008/05/prognationdrummers.jpg