Regirock for B+ rank, or maybe A-
One of the biggest problems with regirock in standard play is the absence of reliable recovery. With that fixed through Regenerator, it has become one of those things that is annoyingly difficult to take down, with 80/200/100 defenses boosted by assault vest gives it insane bulk (not to mention some people even run sand with it for 2.25X spDef). A 100 Base attack stat is not too shoddy, and with quakeslide coverage along with ice punch for grass and dragon types, the power of this thing can often surprise. It's one of the biggest pains for offense to deal with, especially if said offense does not carry a tough claws + fighting type physical attacker.
I think the one thing holding Regirock back from A or even A+ are a) its awful typing; weak to water, ground, and fighting, three very common attacking types. It also pretty much has 1 very good set (assault vest + regen/sand stream), which makes regirock incredibly predictable. Still, even though I don't play stall/bulky offense, I know it's an important part on said teams, and it deserves a ranking.