I think larger sites tend to be best, although with a large membership pool you sift through a lot of people who are all looking for different things. I have seen just about everything on tinder, and have successfully found poly women to date on that platform. I have also dated poly women on okcupid.
Tinder doesn't necessarily "allow" you to sign up as a couple, however profile configuration is flexible enough that you can configure your profile as a couple if you wish. On okcupid you can link two accounts as a couple so you can each have independent profiles, which is think is nice, tinder does not offer that functionality.
Bumble does not seem to have many poly people in my area...
Fetlife has some sort of poly community however that website is mostly directed towards kink and sex specifically.