if you don't mind spending extra candy+dust for bragging rights, because that's all it's going to be in almost all situations, the level 20.
if you just want something you can use right away, go with the level 26 one. the ivs will not make a difference in most fights. the stamina results in 1 extra HP which won't matter in most fights cos your pokemon will either get overkilled or won't get that low when it defeats an opponent.
the extra attack iv wont make any difference in most situations because of rounding in the damage formula. your quick moves will do the same damage in almost every situation, and you'll get like 2 extra damage from using a charge move, which again won't really matter, because you'll probably overkill the opponent anyway,
you'd have to live on 1 hp due to the stamina iv and have the extra damage from the attack iv allow you to only just defeat the opposiing pokemon, for the perfect iv one to actually make a difference.
inb4 ivs affect your cp by like 30, just power it up once or twice and it'll still end up being cheaper because you won't have to pay the cost of powering it up from 20 to 26. it'll only matter if you're maxing it,
the cost of 20 to 26 is 39k and 34 candy which is more than the current maximum power up cost, which is 10k and 15 candies.
