Define best.
I’d say that XF Cloud and Invision Cloud have things to recommend both of them (esp Invision Cloud which has features not available in the self hosted edition) if you’re looking for a more traditional forum experience.
Circle has a lot going for it if you’re aiming for a more social network type feel, and it has video features that the self hosters mostly only dream of. I find it funny how much they talk up their customisation features but in reality compared to self hosting (or even Jcink/Proboards/Forumotion) it’s actually a bit tame. Popular with people who have a diverse community backing where the community itself has little in common with each other - think fan communities/promo sites for singers in bands. (This was my first exposure to Circle was the official fan club of Floor Jansen (Nightwish’s lead singer), FloorWorld.)
I’ve also heard mention of HiveBrite but I have yet to see any in action and yet to fully decipher the extreme corporate nonsense that is their website.