I'm gonna go with Bucky/ Winter Soldier/ Captain America. Simply because his resurrection exceeded ALL of my expectations in originally wanting him to stay dead, to disbelief that Marvel would do something as
ing ridiculous as that, to accepting him as one of the most badass and well written characters currently right now in Marvel 616 Universe. Not to mention him as Captain America is something that basically isn't seen as just someone to replace Cap for time being until inevitable resurrection of original Cap, but as actually his own take with him taking the mantle.
So Bucky, simply because most developed character in such short span of time.