Choco - POF has the largest number of people I think, but possibly a greater proportion of married men trying their luck, dross, and topless men holding fish.
You can find decent people on there, but I found it's difficult to find them and once you do they're so busy wading through the dross as well that it's more difficult to get a conversation going (but I'm coming at it from a guys perspective) So it's free but takes a lot of work. You can screen out those just looking for Intimate Encounters too, which helps.
Match costs money, of course, but tends to have a smaller number of crap profiles.
Guardian or Times, or something like that, has an even smaller number of profiles, mostly in bigger cities, but they tend to be better quality.
OKCupid takes a lot of work setting up a profile and does apparently have a lot of polyamorous people, but it's free (although incredibly full of adverts which slow things down to a crawl) - and you can weed out people by misogynistic/racist viewpoints based on their questions, plus see who's just looking for sex because those are the only questions they answered.
Basically, if you're in London or Manchester or something, you can try GSM or Times (for free I believe?) Otherwise, pickings will be thin on the ground.
I've tried Match, OKC & POF - I've only met people off Match though. The others are just too hard work and full of 'hey babez' messages.