Ramirez and Izanami used to be Top Tier. However, everyone got buffed in the last patch besides these two; so I'm not sure.
Many people told me they still feel like their character is weak (e.g. Audrey), although I noticed a huge power increase, atleast in Jerry-terms.
Basically, you can play whatever you want. Story is easy with everyone, the only difficult part will be EX Third World.
Take whoever you like and enjoy the game while it's still online.