The question is easy (maybe).
Who is the best hero for you, right now, at his or her max potential?
I know this question is pretty subjetive but even when im not very fan of this character, I think the best hero I have ever get is WSothis.
Seriosly, she has everything: she heals almost in every attack she does, she has very high attack and what the hell with her speed? (Mine also is +Spd). Just try to double her… With her default build she is just a monster, not need any inversion. She activates specials for free, she has DC, she debuffs enemies and buffes herself…
She destroyed Abyssal Chrom and others just alone, even when Chrom is efective against armor heroes. She can tank all the enemies at the same time she heals and activates his absurd special. She is like a better version of LTiki. But she is inmune to Dragonslayers effects. She also gets a buff of +5 to every stat if she is > 50% hp (and she will cause she heal every round).
What the hell?
Others close options: Surtr, HMyrhh, LTiki, LIke, DEphraim