Snorlax has been consistantly dominent during three generations worth of competitive battling. I would argue that had it not been for the presense of Tyranitar's sandstorm in ADV, Snorlax's Curse set would've been almost impenetrable bar the odd CBHeracross there force him into submission. It can tank every special attack almost as well as Blissey and at +1, many physically-based suicide attacks fail to OHKO Snorlax as you Rest off the damage. CBMetagross must explode when Snorlax starts Curse up, otherwise prepare to whitness an uncanny comeback before your very eyes.
So, Snorlax's ability to tank hits as well as dishing them out are apparent, but its versatility to run mixed sets are destructively fierce theselves. STAB Selfdestruct is immensely powerful, and the combination of Fire Blast, Body Slam and Earthquake / Focus Punch aim to offset any defensive core early/mid-game before finishing off with a bang.
It's difficult to decipher just who deserves the title as undisputedly "dominent". There are always ways to help deal with these overly common threats more easily, but the one Pokemon who I've been unable to handle at any point of the game has to be Snorlax.
In second place, my beloved favorite, Tyranitar, deserves its merit and potentially is as dominent considering B/W counter-balances the fact Tyranitar was not around for the first generation.