I would say ForumCoin. It has been around since 2012 and the rules are very relaxed which means it attracts lots of members. I have been a member since July 2015 and it is still going strong. Although it's low pay it is my favourite forum.
Another site which many people do not know about is paidforumposting which has been around a while but is very hard to get into. I was accepted about a year ago but decided not to join as it looked pretty complicated. It is a site which tests you on your English skills by asking 10 questions of your choice and then there will be a wait of a few days before they get back and let you know if you have passed. If you can get in it is said to be high paying but I couldn't tell you the rate per post.