I've got an EV question for our resident Chansey experts VaporeonIce and GG Unit. So far, all the big Chansey streaks I've seen use a "max physical defense" spread of 252 HP / 252 Def. Given Chansey's poor base Defense and the multiplier effect of Eviolite, maxing the Defense EVs is obviously correct.
What I'm less certain about, however, are the HP EVs. Chansey's Special Defense is pretty enormous without any boosting at all, which weighs in favor of doing everything one can for physical bulk, but Chansey's even greater base HP mean that the percentage impact of those HP EVs seems quite small, and I'm interested in the possibility of pushing special bulk into the stratosphere instead.
With max HP and Def (and assuming 4 Speed EVs), a flawless Bold Chansey has level 50 stats of:
357 / 22 / 62 (93) / 55 / 125 (187) / 71
With max Def and SpD instead, the stats become:
325 / 22 / 62 (93) / 55 / 157 (235) / 71
Because of Chansey's huge base HP, 252 HP EVs only give Chansey about 10% more HP than an uninvested set, while the Special Defense EVs provide around a 25% boost in Special Defense.
The $64,000 questions thus become 1) Are there any special attackers in the Maison where the extra special bulk would help Chansey, or is she so specially bulky already that this higher special bulk is moot? 2) Are there any physical attackers that threaten Chansey without the HP investment but are more reliably beatable with that 10% boost in HP? 3) How much more common are maison Pokemon that affirmatively answer question 2 than question 1?
Clearly, the HP-maxing spread works fine, as your massive streaks demonstrate, but I'm interested if you had any specific theorymoning or battle experiences that make you confident that HP maxing was a better choice than boosting SpD? I strongly suspect you did, and even if not at the level of specific calculations, I'd love to hear your thoughts.
Either way, maybe I'll play around with SpD boosted Chansey just to try out the alternative.
EDIT: I guess that the relatively small percentage boost in HP from maxing the HP EVs also weighs in favor GG Unit's consideration of moving some EVs from HP to Speed to outspeed Wailord4. So the question then becomes whether a spread of 172 / 0 / 252 / 0 / 0 / 84 is better than a spread of 0 / 0 / 252 / 0 / 172 / 84?
I can't answer for the Maison specifically without sitting down and actually looking through threats, but I know that Chansey is always EV'd with max Def/SpD for OU / Battle Spot as like you said, the HP investment equates to a very small change overall. But that's probably a more 'cover all bases' spread which works better in a PvP environment. Here in the Maison where you know the exact threats that Chansey is used to check you can probably get away with a fully physical HP / Def spread (or maybe even a more specialised spread!).
Imma just let Vaporeon and GG answer, lol.