For those of you who think that Chansey is physically weak too, here is a shocking calc that I discovered (the hard way) today.
+6 Standard Curselax vs. 252 / 252 Eviolite Chansey: 89.2% - 105.3%
That's a STAB base 102 move coming off of a base 110 attack that's at +6, and it won't OHKO most of the time. To reiterate, that's a base 153 power move coming from a 1024 attack, and it won't OHKO a "physically frail" pokemon. Chansey then proceeds to OHKO Snorlax with Counter (yes, many Chansey still use counter). Now this either means that Snorlax sucks (which we know is wrong), or that Chansey is too good.
Now obviously, people are going to say "well that's just a Snorlax", but the point is that when one of the biggest "counters" can't really counter Chansey, something is wrong.
And finally people, let's look at the characteristics of an Uber:
http://www.smogon.com/forums/showthread.php?t=43566
Let's pick this apart.
Offensive Characteristic
A Pokémon is uber if, in common battle conditions, it is capable of sweeping through a significant portion of teams in the metagame with little effort.
+6 Metronome Adamant Chansey (skill swapped pure power) Helping Hand boosted Critical Hit Giga Impact vs. lvl 1 -6 Def Lonely Diglett (0 HP 0 Def IVs):
8342209.1% - 9814363.6%
'Nuff said.
/sarcasm
Defensive Characteristic
A Pokémon is uber if, in common battle conditions, it is able to wall and stall out a significant portion of the metagame.
Mmmkay let's get serious. Now let's look at everything Chansey can wall or stall out. First of all, any special attacker is completely walled, regardless of what pokemon it is. No special attackers in UU can muscle through Chansey. And for those of you who think that sticking toxic on your special attackers will fix the problem, you are aware that Chansey has Natural Cure, right? Chansey's not going to stay poisoned for long, and is going to come back to haunt you. Furthermore, physical attackers without Fighting moves are walled to an extent. Although Chansey may not be able to completely stop them in their tracks, it has enough time to Toxic them and wear them down with Protect, Wish, and Seismic Toss. So unless your sweeper is immune to Toxic, you're getting fucked up. Chansey will stall you out with its amazing Wish+Protect combo as the Toxic damage slowly racks up. And even though many physical attackers are going to get through Chansey, most are going to come out with low health or on the verge of death. So Chansey is capable of walling and stalling out all special attackers as well as weaker physical attackers. And although it misses out on 6% health per turn, it also is capable of gaining 50% in two turns by simply Wishing and Protecting. So pokemon like Hippopotas really can't take care of it that easily.
Support Characteristic
A Pokémon is uber if, in common battle conditions, it can consistently set up a situation in which it makes it substantially easier for other pokemon to sweep.
Chansey absolutely makes everything easier for sweepers. Chansey has an absolutely outstanding support movepool. For starters, Chansey has the ability to pass 352 HP boosts to any pokemon. For most pokemon, that's all of their health. So Chansey is essentially capable of reviving sweepers (as long as they're not actually dead, but instead are close to dead). Chansey is capable of wearing down pokemon, making it easier for a cleaner to come in and wipe out the opponent. Toxic damage and Seismic toss damage really does a number on the opponent, meaning that oftentimes weak attacks can take care of the opponent. Thunderwave allows Chansey to cripple the opponent by halving their speed, therefore leaving them highly vulnerable to moderately fast sweepers, and giving them only a 50% chance at attacking. This makes it easier for pokemon who can't always get the OHKO on the opponent. Parahax allosw them to safely score 2HKO's without any damage. It also allows pokemon like Togekiss and even Dunsparce to run wild. I want to keep this support list short, so I'll simply finish by saying that Chansey can also lay down Stealth Rock, making those OHKO's and 2HKO's even easier for your team. But that is not all. Chansey's support movepool is so incredibly vast, which consistently allows it to set up a sweep for other pokemon.
In short, Chansey certainly meets two of these three qualifications and should be banned under either one.
EDIT:
@ Gerard
I completely agree, Vulpix personifies the support charicteristic