I’d say Balthazar from the season 1 and Bjorn from the season 3.
Of the ones I have which would you feed??
Sorry, I thought you were asking about the best dark 3* heroes.
Personally, I don’t like any of them, but I had to choose, I’d max Renfeld. In case you got the costume, Renfeld’s costume is more interesting than Prisca’s one.
Thank you so much!!i don’t like either of them much but until I find a 4* they’ll have to do 
Personally I would go:
Renfeld > Oberon > Prisca
Also, be sure to do your other 3* before doing 4* and 5*. They use a lot more resources and will take you forever to do. Belith is fantastic so is Ulmer. And if you have emblems to add to them too, even better
Thanks for the reply! Do you think Layla is worth saving?
Depending how many heroes you have. If you have enough 3* to use in tournaments and quests then yes. Ideally good to have 5 of each colour. But if this is all heroes you have got I would wait