I could wax on about dream juggernauts all day, but wasn't there a real thread about this a while back?
In the hardbop vein, Moncur-McLean-Lee-Hutch-Cranshaw-Williams has to be one of my favorites, though if Higgins were to have subbed for Williams I wouldn't have complained one bit (might have preferred it, actually).
O'wise the Miles sextet would do nicely, if I could put Duke Jordan in the piano chair and A.T. in the drummer's seat.
Miles, Trane, Cannibal, Jordan, Chambers, Taylor!
As for avant-garde, Cecil always seems to get pretty great sextets going - especially some of those early ones.
Here's another dream one (sort of): Cecil, Shepp, Lacy, Curson, Buell, Charles. That would be cool.