Which abilities would you give them as their final one? So far they all can have two.
Also, this is mainly from a logical point of view, something like Magic Guard Charizard or Wonderguard Swampert, however much better it would make them at battling, is not a logical choice so try to keep them within the realms of possibility.
I'll start off.
I think Intimidate would be a nice pick for Serperior. Yeah Contrary is better blah blah; however, according to the dex it has the ability to stop foes in their tracks by glaring at them (obvious nod towards the move Glare too), which I also think would make sense for Intimidate, as it's basically intimidating the enemy. It would also mesh well with its decent defenses and switching into physical attackers. Shed Skin is another obvious choice for it; since it doesn't have access to Aromatherapy (normally), it would also be quite useful.