The ones I play the most in no specific order: book of 99, fa fa babies, gonzo's quest, mega joker and Granny vs Zombies. Out of these, book of 99 is my favorite.
Book of 99 gives you the chance of free spins and you have a a guaranteed bonus when you collect 99 books, which normally takes between 200-250 spins, best bonus I had was something around 300x while the worse being something around 20x.
Fa fa babies is also a progression game, you collect some sort of coins that are attached to symbols, to collect them they most form a winning line, 138 symbols must be collected to unlock feature, it takes around 200 spins to get them, it's a sort of jackpot, you pick some coins and picking 3 of a kind gives you the respective jackpot, to top up that jackpot you get envelopes on base game and there are 4 of them, the lowest one will be something aroun 25x your bet to the moment you unlock the feature and the highest one can be up to 1000x or 1500x if you grind enough envelopes, you also can get free spins but they are hard to get and normally don't pay much, but the game has a good hit frequency.
Granny vs Zombies also got a symbol-collection feature, similar to fa fa babies, upon collecting the required amount of coins you get a jackpot. Gonzo's quest fairly regular slot with no additional mechanics besides the multiplier that increases on each win in a row and Mega Joker is possibly the most traditional slot of them all, but I like it simply because it gives me consistent sessions.
The reason why I prefer the symbol-collection ones is because you have a guaranteed bonus, instead of endlessly spinning a slot and get no bonus at all. I will list the slots that got symbol-collection to unlock feature that I know of in case you are interested: desert gem, granny vs zombies, fa fa babies, mida's trasure, dwarfs gone wild, book of 99
Automatic translation: