Which Chansey to evolve and max out?

This is a no brainer. You want the best defense possible, since Blissey's defense is so low, as well as attack. I think it's pretty clear that a 10 stamina 15 attack 15 defense Blissey is better than a 15 stamina 14 attack 14 defense Blissey.

So power up the 98% one with 14 Stamina, and don't look back. And when you have enough candy and dust, power the other 98% one up too - it will also be great.

All that said, the difference in IVs for Blissey is pretty small, and all of them are good.

Not trying to hate or anything, but listen to me: just don't.

I'd really advise against spending anything on defenders at this point.

But if you're going to and want to choose the best one, it's a no-brainer just like aeronaut said. Defense is more important than stamina for both Chansey and Blissey.