Dicky Betts is the rare example of a cat that can do both, rip AND rhythm. Plus it's in his own voice, with exemplary dynamics/melody, and endless invention. Check out the way he sets up most every groove on Evening With The Allman Brothers Set 1, especially Where It All Begins, and South Bound.
+1 What a great album. Nobody knows is amazing on that album as is the version of Elizabeth Reed with that groovy minute long bass solo.
Also whoever said Dave Matthews was a great Rhythm player, I agree. It's a shame it became so cool to rag on Dave...although his newer stuff just doesn't live up to Under The Table and Dreaming, Crash, and Before These Crowded Streets.