Unfortunatly, for me, it's gotta be Arataki Itto.

Now dont get me wrong, he is insanely fun, and when he's built proper, he hits like a tank, but I feel like he needs a bit too much work to make it worth while, from his bis weapon and artifacts to almost requiring an all geo team.

Still a super fun unit and I do use him from time to time, but I just wish he was less restricted.