The sweet spot for me is a group of 6 -- 5 players, 1 GM.
This leads to the most variety of playstyle options, the best inter-party interactions, and so on.
4 players is nearly as good, though for some reason I've found that adding that fifth player character just seems to add a bit of "spice" or "zest" to the proceedings that isn't there with only 4 players.
Anything less than 4, and you've got to have a group and GM with a solid social contract, since it means the players have to be more directly involved, and be prepared for lots of spotlight time.
6 players is manageable, but there's a noticeable drop in characters' ability to generate "face time." Invariably, someone at the table is bored / not paying attention. It actually saps the energy away from the table, rather than adding to it.
Anything over 6 players is just a massive waste of time for the types of games I enjoy playing / running.