I'm not sure I can make ten!
Based on most played, best customer service, best overall experience I guess!
1st
32RED
(By a mile, for the millions of reasons all over this forum)
2nd
Spin Palace
(Responsive C/S staff, few different slots)
3rd
TotalBet
(Who? Tiny little crypto, nice people, quick payment)
Others that haven't particularly stood out a mile, but played there with no problems... AUS Vegas, Maple Casino, Littlewoods Casino, INETbet and more recently (i.e yesterday

) Virgin Games - worth a mention for the great slot selection!
EDIT: Oops, The Gaming Club too! Little bit slow on the old customer services sometimes, but great random bonus deposits and match promotions. Always a 'busy' vibe with plenty going on.