I found the thread on guitarists fun, but as a drummer, this one interests me more personally.
The topic seems self-explanatory, though there are some who will no doubt be mentioned that I’d like to pre-emptively exclude. Ringo Starr, for example: while there’s definitely debate over his skill, there are certainly enough people that appreciate him that he doesn’t qualify as “underrated”. Similarly, someone like Max Weinberg, while not usually brought up in lists like these, I think has achieved plenty of recognition both within the industry and from fans that he can’t reasonably be thought to fit the category. Same applies to, say, Roger Taylor.
Some of my picks (I’m sure I’ll think of others):
Bun E. Carlos (Cheap Trick): Just as steady and basic as Charlie Watts or Phil Rudd (of AC/DC), but almost never mentioned in the same breath. The guy can both swing and pound, often at the same time; non-drummers almost always fail to recognize how difficult (and important) it is to be at this level of understanding of rock rhythmic basics. Key tracks: “Surrender” captures it all.
George Hurley (Minutemen/fIREHOSE): Brought jazz phrasings to punk, and found inventive new uses for the toms. Managed to come up with the perfect beat for his bandmates’ avant garde compositions. More musos should be familiar with his name, but he remains sadly under-regarded. Key tracks: I like pretty much everything he did on the “What Makes A Man Start Fires?” album.
Pete Thomas (The Attractions/The Imposters - Elvis Costello’s backing bands): For my money, the most versatile player in rock, and also the lightest touch. Can perfectly complement any of Costello’s innumerable stylistic exercises with inventiveness and taste. Most people don’t even rate him because they consider him a “backing musician”, but I imagine among musicians he’ll go down as one of the greats someday. Key tracks: “No Action”, “Lipstick Vogue”, “Beyond Belief”.
Bill Berry (R.E.M.): As critic Robert Christgau rightly observed in his '84 review of Reckoning: “As befits good Southerners, their sense of necessity resides in their drummer, which is why the Byrds analogies don’t wash (who ever noticed Michael Clarke?)”. Berry’s drums were as much a part of R.E.M.'s signature sound as anyone else in the band’s contributions, and he, more than anyone, kept the music anchored when a lesser drummer could have easily let it drift off into the ether. Picture the songs from their first 4 albums performed acoustically without percussion and you’ll have an idea of how much propulsion he provided. Key tracks: “Wolves, Lower”, “1,000,000”, “Harborcoat”.
Morris Windsor (The Soft Boys, Robyn Hitchcock & The Egyptians): Another eminently tasteful drummer who never called attention to himself but always found the absolutely perfect groove for each song. Not much to be said; just a consummate craftsman, while being much more creative than that would imply. Key tracks: “Egyptian Cream”, “Brenda’s Iron Sledge (Live version)”.
Chris Frantz (Talking Heads): Made David Byrne’s quirky-jerky songs danceable. Probably the most incongruous drummer ever in terms of appearance vs. sound; if you saw him on the street, you’d think he was a CPA from Minnesota, but on record he (along with wife Tina Weymouth) laid down the funkiest bottom in all the new wave. Key tracks: “Once In A Lifetime”, “Burning Down The House”.
Topper Headon (The Clash): Strummer & Jones are (rightfully) given credit for expanding the palette of punk with their songwriting leap on London Calling, but there’s no way they could have realized such a conceptual leap with a standard four-on-the-floor punk drummer. Everything they did from '79 on benefitted immensely from having someone with real chops in the drum seat. Key tracks: “Lost in the Supermarket”, “The Right Profile”, “Armagideon Time”, “Straight To Hell”.
As I said, I’m sure I’ll think of more later.