Generally, I prefer 5 or 6...in all the DMing I've done over the past 24 years, that seems to be about the average I've had, and it just feels right to me. And, with that many PCs, you're usually assured that the party won't have any glaring deficiencies (unless everyone comes to the table with a rogue, or something).
I've never DMed for fewer than 3. The upper limit for a reasonable game, for me, seems to be around 8 or 9.