This thread seems to be focused on the letters B and S a lot, which I think is BS.
(insert 2 drums and a rimshot here)
In all seriousness, Rampardos. Rampardos has the highest base attack of all non-legendary non-mega evolutions. And now it gets Rock Wrecker and Rollout. When combined with a Life Orb, it can get off some serious damage, but when combined with a Wide Lens (yup, using a Wide Lens) and perhaps switched in on a Baton Pass with boosted Speed and Defense/Special Defense, you can get off all 5 hits of Rollout. The fifth one will have a base power of 480, which, when combined with STAB and the already high attack, can wreck practically anything. I don't have time to go to the damage calculator, but lemme be the first to say holy s***.
EDIT: Sans Wide Lens, the odds of Rollout hitting all 5 times is (9/10)^5, which comes out to...about 59%. Better than Sheer Cold, but that's not saying much.
Speaking of Sheer Cold, Lapras gets Lock-On. 100% accurate Sheer Cold is my kind of Sheer Cold.