In the past I've used Countess Crey as a baseline test, since she's not especially remarkable but still poses an AV-class threat. There are a bunch of reasons I like to fight her for testing purposes:
1) She doesn't have any crazy resists, a tier-9 defensive skill to wait out, or any particular healing ability besides basic AV regeneration, so you can see how you do against just that.
2) She does psychic damage, so you can see how you stack up against the particular challenges that damage type poses. A fair number of defensive sets have a hole to it, so you can see if you're built well enough to plug said hole if you have it. It also does -recharge, which is one of the worst effects to have on you while fighting an AV (or in general...), since it slows down your damage rate and any click defenses you have.
3) Her moveset includes Psychic Wail, so you can see if you can survive an AV-level T9 nova attack.
4) Her moveset also includes a couple status effect attacks, so if your character is prone to mez you can see how manageable those are for you.
5) She's fairly accessible to both heroes and villains in various Ouroboros arcs, and if you feel brave and patient you can see how you do against her and her melee-oriented bodyguard by soloing the Manticore TF.
6) She's a terrible human being.