It's hard to pick a "best" universe, since there are so many different and equally well thought out 'verses, but I'd have to go with Star Wars, Mass Effect, Halo, and A Song of Ice and Fire. Star Wars has a ridiculously fleshed out expanded universe, and is probably the biggest. Mass Effect has an incredible amount of depth to it, with a lot of detailed and unique alien races, not to mention the Reapers' cycle of extinction. Halo has an epic, galaxy-wide scope to it, especially after reading the new Halo: Codex novel. Seriously, that book opens up the Halo Universe tenfold; there is so much we still don't know about the Forerunners and even the Precursors. And A Song of Ice and Fire honestly reads more like a historical fiction, than a fantasy series, because of all the people, places, and events both past and present in the series.