What should I do with my Chansey candy?

Focus on making your best Blissey a super tank. Mid CP Blisseys get crushed pretty easily by Tyranitar and Machamp/Cross. Also, it's tough to pick one movepool. Hyper beam is better general defender set, but Psychic is better at KOing the fighting types. Up to you though.

Despite having cp become irrelevant for gym placement, having a high attack Iv may be ideal in general; blissey has nearly 2x as much defense as it does attack, and nearly 4x hp. Proportionally, each attack Iv subsequently becomes more important.

I'd invest in the best attack Iv one, albeit may not be candy efficient, and then work with the moveset from there. If you're low on candies I'd invest in your lv20 Blissey