Bitterblossom for me. It's one of those rare cards, even in cube, that consistently dominates a game all by itself. It plays in a lot of different strategies, and enchantments are probably the most difficult type of permanent to answer.
Jace, Witness, and Pack Rat all stand out to me, but I think Jace has the most raw power and will make any blue deck. I can reasonably hope to wheel the Fractured Identity putting me in UW control or UW tempo, which are both fine places to be.
Taking Jace and wheeling fractured Identity would feel so rad. Taking jace and getting izzet charm back seems pretty sweet too.