Well I wouldn't really recommend microgaming, their slots pay only about 95%. Although if you are going to, you could do worse than sign up with the Belle Rock group as they all have slots signup promotions.
Harrods slots are much better and pay out >98%
Wagerworks casinos are also excellent, and all have posted payout percentages, which AIR, are pretty high, definitely well over 95%, but don't remember the exact numbers
Careful where you play as some slots payouts are even worse than microgamings
I would avoid realtimegaming slots as their software is totally tweakable, which basically means that the slots could be paying out 80% and you wouldn't know.
Cryptologic casinos also have low payout rates.
I would probably go for a Wagerworks casino over the rest, as their games are nice licensed properties, which should appeal, such as Baywatch, Wheel of Fortune, The Price Is Right, etc.
And all with posted payout % too, but take care, as it varies with coin size - they often take more money at lower coins (although this might not be a bad thing, as a higher percentage of less money is probably elss money
).