I bought several rings off eBay until I got one that fits. It is probably half size bigger than my usual ring size, because for most of the tricks you need to have a ring that can easily come off the finger, though, it should still not fall off when you keep your hands down in a natural position. I'd recommend thin band ring, the colour is up to you, but I like black, because of its nice contrast with colour of the skin. Obviously, if you're black, a silver ring would be a good choise.
No stones or bulkiness on the ring though - if you want to master Zach Heath's routine (which I'm working on right now), band ring is the only option.