My favorite is...

Finisher- Elemental attack
Honestly, I got that more than anything else.
Though I did die a few times because i healed the monsers, but most of the time, I just came back and used the right character for it

My most used or needed ability would be Lainuki or however you spell it... All enemies die lol
Also, the gill toss because I had too much, instant 9999's


Worst would be for me... death spell...>.> Oh wait... not an ability
Worst would be... Berserk... Pointless...