Snorlax is really strong, and very few pokemon can safely switch into his Body Slam: Cloyster is one of the best options, but it hates being paralyzed 30% of the time. Gengar has to fear Earthquake; and Rhydon has both of these problems at once. The safest switch-in to Snorlax is often Snorlax itself - this is because Normal types cannot be paralyzed by Body Slam (it's one of the quirks of this generation) so a Reflect+Rest Snorlax is virtually unbreakable by an enemy Snorlax, albeit a bit passive. But as we just explained, even Body Slam by itself is rather hard to switch into, so this set is still quite threatening. If you are not running a Reflect+Rest Snorlax to wall the enemy Snorlax, then you still likely would appreciate one of the many offensive variations of Snorlax (4 attacks, 3 attacks + Counter, or various Amnesia sets) as they all amp up Snorlax's threat level to eleven and they can often generate 2-for-1 trades if set up in the right conditions.
Chansey is simply by far the best answer to very many Special threats; she has access to the potent Thunder Wave + Soft-Boiled combination, which allows it to outlast virtually anything in a 1v1 (even some Physical threats!) as long as they don't have recovery themselves, since Chansey can simply paralyze them and then spam heal until they FP. She is rather toothless offensively, but she still has the tools (through Seismic Toss or the notorious Ice Beam+Thunderbolt combo) to force the enemy to reposition and/or spend turns healing, at which point she can safely switch out to more threatening teammates. What sets Chansey apart from other users for the Thunder Wave + Recover combo (Alakazam, Starmie) is just how hard she is to kill: Alakazam's physical defense leaves much to be desired and he is easy pickings for Snorlax (whose Body Slam kills in 2 hits), whereas Starmie has to contend with an Electric weakness that leaves her exposed to the various Thunderbolt users in the tier. However, almost nothing can kill Chansey in 2 hits - Rhydon is the closest but he is himself 2HKOed by Ice Beam, and naturally slower. The Normal-types can get there with Body Slam + Hyper Beam, but obviously the Hyper Beam is contingent on getting the correct read on whether the opponent will stay or not. This means it is extremely hard to break Chansey out of her TWave + SoftBoiled cycles, and it reinforces the most reliable answer (paralyzing the Chansey and then switching to Snorlax) as a dominant metagame force, too