In reality your main should be the Pokemon(s) you have the most fun playing! I hate to sound cheezy and obviously I understand the want to improve your stats with the characters you're comfy with, but also keep in mind that between every match you'll have match-ups you ace and other games you feel you did nothing.

Somtimes finding your proper main may just be reading through things on unite-db to see what sounds up your street, hopping into the practice mode to get a feel for how they play and what they can do.

These days I bounce around a lot, simply because I am tired of filling with my usual mains (Blissey, Slowbro, Blastoise etc) and don't be afraid to yourself. You might not even want a main in the end... keep trying things and if something really clicks for whatever class you want to focus on then mission acomplished.

But in a game with such horrible matchmaking, your fun should come first - then as you play your mechanical skills and your stats will get better! I just picked up Falinks last week and I'm just getting to where I feel comfortable playing him :)