So for me it's kinda different then to a lot of people...
#1 Quickspin, by far, I have made the biggest wins on Quickspin slots, 1200€ on Royal Frog, 555€ on Second Strike, 480 on Sevens high and loads of big wins (2/300€) on other ones with The Big Chase as absolute favorite, always playing a maximum of 4€ a spin
#2 NetEnt, of course, Jack and the Beanstake is my favorite, I love the graphics, the gameplay and the music with the walking wild, and have made some very high wins on it, but also Gonzo's quest, Secret of the Stones, Jack Hammer 1&2 and loads of others I like to play, I like their gameplay, the sidegames and most of times I can predict a little if free spins or big ones gonna drop.
#3 Thunderkick, fairly new to my top list, but Esqueleto Explosivo, 1429 Uncharted Seas and Pink Elephants have made me some good money.
#4 Yggdrasil, started my online gambling on Vikings Go Bezerk, I went almost bezerk myself not knowing about withdrawals and other systems in the online gambling world, but as with most of their games, the graphics and music is beautiful, the quality is very very high, but also it can eat money like no other and that's why I tend to play them sparsely...
#5 Microgaming, of course, Immortal Romance, do I need to say more?
Others like, Lightning, Play 'n Go Elk Studio's etc. are also on my fav. list, but they won't get me going like the ones mentioned above
@cocopop; what's so good about 7 Sins?
The payouts when you are in freespin mode or the double 7's made 400€ with 5 of a kind& a double 7 playing 5€ a spin