In my experience and a ton of theorymoning, the best Chansey spread is probably 160 HP / 252 Def / 96 SDef Calm. In particular it takes 2 physical hits that Chansey should be able to take rather well: it is never 2HKOed by specs Latios psyshock (no rocks, but max defense dies if rocks are up to) and it is only 2HKOed by Tornadus's superpower 6.25% of the time after rocks.
Outside of that you should be utilizing Chansey's amazing bulk. In my opinion, if you go for maximum defense, you just end up with a slightly more specially defensive Blissey, that unnecessarily can take physical hits well, and has no leftovers, making it overall a worse Blissey.
Also note that the smogon Chansey analysis is almost 2 years old now and has counter as a set and has stealth rock in OO...
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