Chansey evolution dilemma

Attack all the way. Defense and stamina aren't nearly as influential. A Blissey with trash IVs but perfect attack will still have high cp. not that I would invest in one like that

Blissey's weakest base stat is attack; so that is by far the most important IV. The stamina IV is barely relevant due to the extremely high base. Defense IV is only marginally important.

I'd happily trade you one of my 90+IV Chanseys if I could for a Rhydon w/ MS /Eq or SE

Attack by a long shot, both in terms of CP and actual combat ability. For reference, 2 points into attack give more max CP than 15 stamina, 4 attack about the same as 15 defence, so 6 points into attack about the same as 15 in both defence and stamina.

Attack IV is so disproportionately weighted that total IV is less important, while the distribution is very important.

Chanseys are rare enough that even lower IV ones could be still worth powering if they're decently high level and/or have good attack. There's no telling when you'll get another Chansey, and whether or not it'll even have better IVs, if you'll even get another one, so if you've got enough candies to evolve and power up a Chansey to where it'll sit comfortably in gyms, do it and start earning some extra coins.

I got a 14/10/10 75% chansey, I still hold onto her but I'm close of pulling the evolve trigger. Even being a 75% IV pokémon she as 87% of the max cp because atk is her best stat.