Don't know if they accept Bulgarian players but
EUcasino is a topgame casino, so they have the marvel slots. I'm not that big a fan of topgame casino's but really enjoyed playing there. And class1 would be my 2nd choice.
Personally I like the microgaming 32red group.
32red, golden lounge, dash and nedplay.
Always feel like I have a good chance of winning even when I don't win
And another favorite is the casino rewards group. Like for instance Blackjack ballroom, captain cooks, casino action. Their freeplay promotion is awesome.