Depends. Amara is definitely the objective 'strongest class' at this point - and generally she has the best COMs and anointments. Fl4k isn't quite as powerful and is more difficult to play at M3/4 - but I do find him better designed overall. Both are still great and I think you will have a blast with both - they are much more versatile than Moze and will allow you to experience the game in a new light.

I was trying to make the complete set of 4 characters in the comments

That is literally what op just beat the game with

Soooooooooo I just beat game on Moze. Should my second be Amara or Flak?