All duplicates (There’s enough varity to not need to keep those just yet) and any S1 hero who isn’t already maxed (They’re the easiest to get back). Work on whatever is left.
Gormek with his 2 Costumes is a superstar!
I’d keep a copy of every unique hero! You will need a deep roster for elemental quests and towers (and tournaments). I would suggest you buy more roster space!
I couldn’t see any duplicates but I suggest keeping some duplicates e.g. Elemental Defence down (EDD) heroes like Falcon and Almur also some healers (e.g. Melendor, Sabina, Richard etc until you get a deeper roster)
And agree c-Gormek is a superstar
Keep at least one copy of every hero.