I always find it incredibly hilarious when people post things like "Hey look guys these physical attackers with over 125 base attack can usually 2hko Chansey when holding a boosting item! It's not broken!" I see posts like this every time, and I expect to see a lot more of it before we boot that fat bitch back upstairs.
No, it actually doesn't matter if a couple of absurdly powerful physical attackers can beat Chansey, because defensive suspects should not be evaluated in the same manner as offensive suspects. When you say "heracross isn't broken because it's countered by gligar / nidoqueen / crobat / chandelure / whatever" you are making a good argument because Heracross' job is to destroy a team of Pokemon, and being stopped by a lot of Pokemon means it can't be too good at its job.
Chansey, on the other hand, is a special wall. That is its job. You can't say "chansey isn't broken because it can't wall absurdly powerful physical attackers" - that's like saying "staraptor isn't broken because it can't take ice beams".
As a special wall, Chansey is good enough to shut down damn near every special attacker in the tier. Of the top of my head, the only common / viable special attackers that can beat Chansey are Chandelure, Togekiss (not even a fair example because this thing just shits on everything defensive ever), Mismagius, Mew, Azelf, and...uh...that's it. Even crazy shit like CM LO Raikou with Aura Sphere handily loses to Chansey one on one (+1 LO Rash Raikou's Aura Sphere v. 0/252 Chansey: 30.8% - 36.4%, and if you wondering, the same easily 2hkos every other special wall in the tier). What about something like Cobalion? It has fucking STAB Focus Blast, surely it should be able to get past Chansey? Nope - +1 LO Cobalion's Focus Blast v. 0/252+ Chansey: 43% - 50.8%.
Chansey is so good at its job that its very existence will cause the metagame to become centralized around physical attacks. When Chansey was around, I ran a team with 5-6 physical attackers, because being walled by Chansey means that any game against a team with Chansey has several of your pokemon being nigh useless. If it drops down again, and for some reason we don't get rid of it, I'll definitely do the same thing, just like I ran Regirock and Impish Rhyperior during Staraptor meta. Should I have to do that? No. No I shouldn't.
And as a side note...I emphasized earlier that Chansey was a special wall, so basing arguments around its ability to take physical hits is missing the point. However, I do not want it to be thought that Chansey is frail. Quite the contrary...
252+ Rhyperior's Earthquake against 0/252+ Chansey: 39.3% - 46.3%
CB Weavile's Ice Punch against 0/252+ Chansey: 35.7% - 42.3%
Max attack (leftovers) Honchkrow's Brave Bird against same: 43.2% - 51%
Max attack (leftovers) Escavalier's Megahorn: 45.9% - 54%
Oh and just for posterity:
Defensive Hitmontop's Close Combat v. 252/252+ Chansey: 46.9% - 55.4% "lol"