For me it’s Casumo at the moment.
I used to love Videoslots, they easily have the widest range of slots, but with the whole reduced RTP thing and the poor customer service, it is just no longer appealing to me. Which is a shame because their reward selection (e.g. Weekend booster) is excellent.
Grosvenor has great customer service and the deposit match bonuses are the most competitive I’ve seen! But their range of games is more limited than the others.
Casumo for me is the best overall. They have great customer service and an excellent selection of games. Very clean, fast website. Their rewards aren’t the best, but they do regularly (at least for me) offer decent deposit match bonuses.