re suspension seat posts, the Cane Creek Thudbusters (and the no longer available Moxie) totally rock. absolutely no sloppy twisting of the seat, it just goes back and down, which keeps your leg-to-crank distance relatively constant.
if you have sufficient seat post showing with your normal riding position, get the LT version. if you're on the heavy side, put their
#7 or
#9 elastomer on the Thudbuster.
The most comfortable cushy seat I've tried is the Giant Unity I put this on my wife's bike after she complained about nearly everything else we tried. I avoid springer seats as they tend to roll from side to side while you pedal, very disconcerting... This Unity has rubber elastomers instead of springs, and doesn't do this at all. But, grrr, of course, its no longer made.