At number #1 is, of course, Humble Bundle
https://www.humblebundle.com/ Every couple of weeks they come out with a big bundle from known developers. They are not the cheapest, though, But definitely the best.
My number #2 pick is, Indie Gala
https://www.indiegala.com/ indie gala isn't out to get you great deals or AAA games. But they are there to support indie developers by helping them to sell their games.
Number #3, Bundle Stars
https://www.bundlestars.com/en/ Bundle stars have some very good bundles sometimes Though they do like to rebundle games, so if you're like me and you buy many bundles. It's often worth checking if you already own a game that has been bundled.
Those are my Top 3 Bundle sites, Steam has also started allowing bundles so we will see what comes out of that.