Um.... well. The dating threads may be useful here.
I've tried a few. To be honest it depends where you are in life. My personal reactions might not be yours.
Always assume every man is one step more married than he says - if he says he's divorced, he's probably separated at most; if he says he's separated, he's still married; if he says he is single but relaxed, he probably has a girlfriend; if he says he is desperate and lonely, he may just be single 
; if he says nothing very much, assume he's married.
I was on one site for a very small niche interest - painfully slow and few contacts but I met two people on there who have both been nice to spend time with.
I was on Guardian Soulmates (RIP) and was scammed. Beware widowers, they are usually scam profiles apparently.
I was on Match which I didn't like - it felt stressful having to reject or like lots of profiles. I did end up going on one date which was bearable and chatted very intensely to one guy over 36 hours which ended up in a car crash of an argument! It does kind of make you do stuff.
I looked at Bumble and Tinder but just found them too stressful, came straight off them again but lots of people have success with them, particularly if you are in a city.
I was also on a couple of hook up sites which I assume is not what you are looking for. Great fun and more relaxed than actual dating sites, and I'm still kind of seeing one guy I met on a hook up site 18 months ago. A lot of men on there are in fact open to some kind of relationship or at least FWB but a majority will be married.