well, based on your account's trophy range and amount of brawlers you have and their levels (and my personal experience), I think you should max out Jacky, Gene, and Jessie for various reasons....
1. Jacky is so OP in brawl ball at 600 trophies that its NOT EVEN A JOKE (also in solo/duo showdown too)
2. Gene is currently the best support/aggro brawler in the meta right now due to his high damage with spirit slap...also, he DOMINATES in Gem Grab, solo/duo showdown, and Siege as a healer and as an aggro brawler (he's only really a support brawler if you max him out and get magic puffs)
3. Jessie is like the MOST UNDERRATED BRAWLER! I mained her for the longest time on my account and i got really good results in Brawl Ball, Heist, Bounty, Siege, and Gem Grab.....If you get shocky as your first star power for her, you are set already to get her to rank 22!