My top 3 providers are...
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Merkur / Bally Wulf / Gamomat - These games are fun, but tough to win on and thus I enjoy them from time to time
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Now that you mentioned Merkur and Bally Wulff, I realise that I forgot "the Germans".
In that case, we could also add Amatic.
There is also EGT, n°1 in Bulgaria, land based and online. They are quiet big in Eastern European countries.
Casino Technology is also from Bulgaria, n°2 after EGT.
We could also have a look at No Limit City, only 4 games released so far, Space Arcade, Creepy Carnaval, Sushi Mania and one not really worth mentioning, very talented ex-Netent guys backed by ComeOn, Cherry or whatever they call themselves lately.
Just For The Win (I told them I thought they had a funny name), also ex-Netent guys, including Henrik aka Tiger, the game designer behind Spinata Grande, Guns and Roses, Princess Koi but also Sparks (nobody is perfect), the ex CPO of
Leo Vegas and other amazing guys. They've released one game so far called Goldwyn's Fairies (very interesting returning Wild feature) only available through the Quickfire platform.
I'd also recommend some games from Red Rake Gaming (seen earlier on this thread), they are based in Valencia not far from us (we're in Barcelona), such as Wildcano which is quiet innovative in terms of feature and Super 7 Stars (my favourite).
I'd love to see what Simmo has to say about the topic.
And I didn't vote yet...