Like they've said, it depends on what you're looking for.
My much younger partner hipped me to what's going on amidst the dating apps, which she enjoys browsing, if only for new friends and the occasional possible date.
- the straight person's version of Grindr, a gay hookup app. Tinder is basically an app where you make a choice whether you think the person is "hot or not", meaning hot enough to meet. If they are a mutual match, y'all can start chatting. The unspoken rule of Tinder is that it's for hooking up/sex, after a few casual dates first. Beware though, youth and beauty are required to be successful. Or at least a picture of a dog in your profile
- again, a young audience, but more of a relationship seeking app.
Plenty of Fish
- much older, more world-weary audience. You're going to find members here who are mostly 30-60s, have at least one kid, and who are looking for something long term. Nothing wrong with any of that of course.
Skout and DateHookup are kind of a mix of OkC and PoF, but with much fewer users.
As for poly-dating specifically, I'm not the expert. Sorry.