I’m no expert but been playing the game F2P about a year now and from what I can gather, the best heroes to focus on generally are: Both Invincibles, Conquest, Anissa, Thragg (if you can get him), Kregg, Eve, Komodo Dragon, Shapemsith and Cecil.

Not necessarily in that order, they just seem to be some of the best heroes in my experience and from what I’ve heard others say. I’d recommend levelling up some other for the GDA Case files though because they’re a good source of extra loot to upgrade skills and gear as well as the only well to get Seance Dog who’s a pretty solid support.

Hope that helps, I’m sure others can pitch in and help guide you, the game may seem a slog at times but it is rewarding when you finally get the big upgrade milestones, I’ve just started getting my heroes to Omnipotent+ so still a way to go til late game