Not counting Dimitri and Byleth who are powerhouses, I'd say Felix is my best offensively, so it would have to be between him and Mercedes who is a great support. Now Ingrid, on the other hand... I've been babysitting her pretty bad and she is still kinda weak. It's been better as a Pegasus but there are no advance classes to follow up on it, which is stupid tbh.
Also, what have you guys made your Leonie?