In RBY and GSC nothing but superlatives describe Snorlax. Briefly, Snorlax in RBY is the best (or at least, the most reliable) wallbreaker and the most versatile Pokemon in the game. In GSC Snorlax is even better at offense and more versatile AND is the game's best Special Wall to boot.
Snorlax is not always walled by something in GSC. Most teams prepare extensively for it, true, but it still smashes them with a Drum set or a Curse + Fire Blast set (most walls take heavy damage on the switch or don't run recovery) or a SelfDestruct set or an LK set. It's impossible to always have Snorlax walled, and if you can always have it walled you're going to have gaping holes to other things.
In ADV it lost some effectiveness with extra hazard layers, buffed Leech Seed, more prominent Normal resists, the new EV system, and Sand everywhere. In spite of all these nerfs, though, Curselax can still easily tear through an ill-prepared or a weakened team, and straight-out attacking sets are pretty scary, too.