My big 3 are Pond5, Dissolve and Story & Heart.
A relatively new one, Filmsupply (sister site of The Music Bed, which I use a lot), is great because it actually features footage from known filmmakers and shooters, like Philip Bloom, for instance. I haven't yet used it, but a lot of the footage looks beautiful.
Pond5 definitely has the best selection and prices, and I often go there first. They also pay the content producers more - I think 50% - so that's cool. They also have an extensive array of Public Domain (totally free) footage, as well as templates, photo, 3D models and digital backlots which are really interesting!
Dissolve has a really nice interface, search engine and wonderful curation and research services in which you can chat with a rep about your project, and they will jump on board for free and come up with playlists of videos which may work for you. It's really helpful, since they know their collections better than we do! Sometimes, by the way, I'll find a clip on Dissolve and copy-paste the clip's title into Pond5, and find it there for a better price. Dissolve also has a lot of vintage footage, and a full selection of stock photo.
Story and Heart is a truly unique site. Rather than just organizing footage by keyword and producer, their clips are uploaded in groups called "stories," which are visually cohesive, often from the same exact location and shoot day. You can really use this site to create a journey in your piece. Their search engine also let's you refine your results by color and energy level too. They're pricey, but once you see what they offer, you'll understand why. Very fancy.
I hope this helps. I've been using a lot a stock in the past couple years, so I'm well acquainted.