This. Gigant. And. Round. Thing. Hits. Like. A. Truck.
With the difference that a Truck can't beat it 1vs1. That's pretty much says all about Snorlax, he can beat almost any Pokémon on a 1vs1 that doesn't have Fighthing STAB and later just Selfdestruct itself, or Rest the damage and continue it's onslaught.
On the first generation, it's one of best Pokémon over there thanks to a monstruous STAB Hyper Beam and Body Slam, even when it can't wall things all the day, it can resist some hits with enough luck, and defeat anything on a 1vs1 with a bit of luck. It's problem is the fact that Gengar and Rhydon/Golem are really common, so, it can't Selfdestruct itself so easily, Rest is really dangerous on RBY thanks to the huge number of secondary effects and Critical Hits, and the prevelance of sleep (they can just try to sleep Snorlax when it wakes up), and can even lost some 1vs1 thanks to it's really low Critical Hit rate and the higher ones of it's opponents. It's still a great Pokémon, and when it can't hit you all the time 2 K.O's or clean like others do, it still can almost always hit at least 1 K.O per match, help absorbing the sleep, as it's good at it, it can also set screens for it's friends and make a surprise counter, if needed. It rarely/never is a dead weight for the team, that's it's great asset on RBY.
On the second generation, it winned a superb special defense, and Curse/Belly Drum, it's offen called the King of GSC, and it may be true as it can really wall and beat on a 1vs1 any, but any special attacker, and can win against any curse user on a 1vs1, rest/resttalk, and get ready for a new K.O, and when it doesn't have any hope to continue, it can just selfdestruct itself and score a second, third, or fourth K.O. It can even sweep and steamroll any team totally unprepared for it with just Curse or Belly Drum. For the luck of it's opponents, it somewhat suffers of 4 slot syndrome, thus, once you know it's set, you can check or counter him without any major problem, then problem is to figure what set it does carry, as if you figure the set too late, you'll at least lose 1 Pokémon by it's bear-like claws. Figthing Pokémon are now good and even great on this metagame, and all of them can check Snorlax pretty badly, like any Hazer and Phazer can do, too, so, it's far from being broken and you can play around it, even if you don't use it on your team, but it's without doubts the best Pokémon of GSC, and every team should be prepared for it.
The third generation were good times, too, I think the ADV Snorlax was better than the RBY, thanks to it's defenses and new toys, and even when it's nemesis were better, Snorlax had all it needs to almost always score a K.O (usually 2) on this era, so, it's still like it's RBY part, just with new toys and counters. This Snorlax almost never had problems to support it's team while scoring always a K.O or two, and that's most than enough to make it a prime OU; still, as it had now some more counters and checks, it does now needs a bit of team support to take care of them, but a prepared Snorlax can overcome almost anything with the enough luck.
The fourth generation... This thing hits like a truck, it may have now rivals for it's position of "I'll always help your team" with the likes of Scizor now being more powerful, but Snorlax is still a prime force, as it can steamroll any team without STAB Figthers or Close Combat users, it does need a bit of support, but if you have a free switch, some support for the Figthers, and some of paralize support, Snorlax will sweep and nothing will stop him. Even without support, it still had it's great power to almost always archive a K.O or two, with just a Choice Band this thing hits so hard that nothing without an inmunity can switch-in at all, and even them should fear for it's coverage and other sets, as Stockpile + RestLax can overcome anything with just a free turn. It's asset is still the same; it shall never be a dead weight for your team.