What should be my go to unit be?

Ryoma, Reinhardt, Fae and Brave Lyn make a solid team for now. You'll probably want a stronger antiarcher eventually - but this has a great player phase and should do alright in AI hands (no team does great with AI).

Personally, I have an Olwen that gets 44 atk and 30 speed with life and death 2 and lance breaker, pairing her with a second unit with reposition. She wrecks most units on player phase and gets moved out of the way. Reinhardt would be good too. She's been my main and has 9999 sp. Reinhardt won't quad hit, but he has higher atk