I'd say Titania is my favorite early-game pre-promote. She's a straight-up force of destruction in the early game, and the fact that she almost always doubles makes her a force to be reckoned with even in the mid-to-late game. She's also different from most Jagens in that she is actually viable through endgame, so you definitely want to feed her kills every once in a while to keep her levels in line with everyone else.
And even though you spoke of gameplay when you asked the question, her character (in addition to her being a great unit) makes me like her that much more.