I'd like to throw my support mainly behind Magic Mirror. Being able to come in on just about any support/status move not only gives a lot of free switching oppertunity's, but more importantly, reflecting those moves back to the opponent is immensely useful in terms of gaining momentum, forcing support pokemon to switch, etc.
Regenerator is a definite solid choice as well, for it to be able to pivot without having to worry much about being worn down too fast. Also allows for scouting more often.
Intimidate is a given, and imo outclasses Unaware, for the simple fact that some pokemon can hit extremely hard from the get-go. Getting them down to -1 means that CAP1 will have such an easier time switching in versus physical threats and forcing them out at the same time.
Prankster is the only other ability I somewhat like. Being able to throw whatever kind of +1 prio support move up after switching in goes a long way to maintain momentum.
Of course, should we chose Prankster, we need a good and diverse support movepool.
Moves like Reflect or Light Screen can help this pokemon become a good check to the pokemon it's currently facing, although that's poll jumping. A move like Taunt stops walls and Encore stops set-up pokemon. The possibilities are endless and Prankster is likely very good for maintaining momentum once you get in.
However, unlike Intimidate or Magic Mirror, which also provide far more oppertunity's to switch in, or Regenerator, which gives some failsaves against faulty switch ins, Prankster really only helps on maintaining momentum, and I feel it is just a little bit worse of than the above 3.