I predict that Dead or Alive will be the most common answer here
€225 for 5 scatters on a bet of €0,09 is something to behold. Free spins with sticking wilds can generate even crazier amounts, I think I saw one of around €800 on that bet
Microgaming's Playboy is also capable of crazy hits on running wilds & wild night features, so are some other 243-liners (Thunderstruck 2, Immortal Romance, e.t.c.). Game of Thrones seems to be capable of the highest return, I think it can pay 20,250x stake.
I also enjoy Playtech's Irish Luck (Great Blue and Penguin Vacation are basically the same game but with different amount of lines, I just prefer the theme and it always paid better for me). It should give 150,000x stake for a full screen of wilds in free spins at 15x multiplier, or at least us hopeful players think so
Maybe someday we'll see it
Secrets of the Amazon is also a game I played a lot recently, in free spins each spin the multiplier increases by 1 and the spins can be retriggered up to potentially 50 spins, with the last paying x50 in such scenario. With 5 wilds paying 50,000 coins that should amount to a 125,000x stake win.
Finally, from time to time I play 300 Shields, even though the paytable sucks which makes it a kind of game where you have to hope for a good bonus round or else you're bust. I just can't resist the potential 300x multiplier