Probably all time I'd go with Fedor Emelianenko as the best ground and pounder, but the top three right now are as follows:
Jon Jones
His ground and pound is so devastating with big elbows and punches. The force he gets from those shots can and will break bones in people's faces. If he gets you to the ground it's either a cut or a stoppage via TKO.
Rory MacDonald
Rory has the best ground at pound at 170. He is very dangerous once he gets a takedown. His last fight on the feet with Ellenberger was safe, but on the ground his punches and elbows are vicious. More stoppages on the ground in the future from MacDonald
Mark Munoz
Munoz generates big power and really winds up into his punches from up top. Kudos for Tim Boetsch in surviving those shots in their last fight.