here we have a graph showing the top 20 games by distribution and no surprise to see Starburst (NetEnt) at the top... almost everyone that has ever played slots has played starburst at some point.
Gonzo's Quest (NetEnt) is still there in top 20 too. and a couple of great slots by Microgaming too (Thunderstruck II & Immortal Romance). Just one slot from BTG is their most successful slot ever Bonanza.