Whats your ideal gaming group size?
For me, its 5.  6 is already pushing too many players in any game other than Amber; while 4 is ok, but if more than one person miss a session you can't play anyways.

With 5, even if 2 players miss a session, you can still run a game.

I can play with 3, though it requires that all 3 players be very committed to coming.

RPGPundit

1 GM and 4 players. 3 players is workable. 5 players starts to get out of hand, as you get lots of off-topic banter and diversions during rule look-ups or when one or two PCs are the focus of the action.

Four players and one GM is my preference.

I'll happily run with lower numbers, right down to one-on-one, but with anything over five players I feel like the game loses focus.

I find that 4-5 players gives the optimum level of inter-player interaction for me. If there's 3 and less players the players don't bounce off each other quite so much, and so end up discussing things really quickly and then waiting patiently for the GM to tell them what happens next as opposed to getting into any interesting IC discussions. There's also too few PC agendas and backgrounds for me to riff off - I find I have to work exponentially harder to come up with additional stuff to keep the game moving.

6 or more players I have the opposite problem: there's so much inter-PC discussion that play slows to a crawl unless I'm constantly having people with guns kick down the door, and there's so many PC agendas and background elements in play that I can't really do them all justice: either I end up focusing on a few and effectively forget one of the PCs entirely, or I try to cover them all and end up not treating any particular element with the care and attention it deserves.

4 to 5 players is the sweet spot for me.

A few years back I once had to GM a scenario for 6 players using 'Battlelords of the 23rd Century' - We may finish figuring out the 1st combat turn by the 23rd Century.

I once ran a Dragonfist game using 3 players that always ended 2 vs 1 in arguments.

I've GM'd Traveller using 1 and 2 players as a teen and that was OK but I prefer 4 players - but it needn't always be the same 4 - my FtA! crew take turns GMing.

Five is optimal, I can manage 6 and all have fun, but more than that is clumsy and difficult, and works only with a few very limited and controlled concepts. (I considered running a Wizard's Duel/nee Pokemon inspired game and have players be wizards AND their pets, that worked well for a slightly larger group, since they paired up to work together anyway.)

Less than 3 is alright but not optimal-- I've run solo games but prefer multiple players.

4 is optimal for me for a one-shot.

It's been too long since I've run a campaign, but I'd say four as well.

Five is the sweet spot.  For a while there I was running a game with eight, which was just ridiculous.  Fun, but ridiculous.  Ridiculous.

4 players and 1 GM is perfect for me - all niches can be filled, there's choice and variety for intercation within the group, but everyone still gets a big chunk of the attention and action.
5 players is OK, but can be tough to handle in more complex systems as Shadowrun, while 6 is the absolute maximum to me and demands a relatively simple system as FTA or Cthulhu.

In my first years as gamer, it was 2 players and 1 GM. As we played most of the time Shadowrun we had probably more diversified and "multi-classed" characters then the designers ever intended, but it was fun to have such weird combos as Samurai Chameleons and Riggin' Summoners to be sufficiently diversified to cover the needed niches.
The good point was that there was much attention for everyone, much room to do your own thing, and it was definitively true to the genre where double teams are the standard (Johnny Mnemonic and Molly, Case and Molly, Cowboy and Sarah etc.)
It's an interesting experience to have such a small group, but I wouldn't want to have it as default.

With a new group, 5 is my max, 4 is ideal.  However, the group I have now is very focused, so I can allow 6. This usually happens on a temporary basis when a friend comes to town, and he or she's usually eager to game so there's no disruption.  

But, I like 4.  I like designing adventures based on weac character's unique skills, abilities, and subplots, and this allows me to give each one screen time on a fairly regular basis.  

Pete

I like three or four, plus me. I used to run regularly with six, but these days I like it more intimate.

Depends on what I'm running. I can run hack and slash for as many as twelve (I'm a freak) but... for anything other than fighting, that's alot to keep  track of. I'd say five or six in that case.

Not counting the GM: 5 is my sweet spot; 3, 4, and 6 are all OK too.  I don't run for less than 3 and I've tried up to 8 at once and that lasted all of one session (insanity!)

With my current thing of short (6-18 sessions), closed-ended campaigns, with 1 or 2 of the players changing between campaigns, 4 is what I've been going for. With the pool of 20 or so players I've access to at any time, 3 is actually the most I can have and be sure everyone's on the same page in terms of game play style. But because you never know exactly how the campaign will go, you need 4 to make sure you get those 3. Also, if any of the players aren't able to make it, a session that usually has 4 going to 3 is no sweat, 3 going to 2 can be a bit harder.

So I'm a bit torn at the moment whether 3 or 4 is the best number for my GMing and preferred play styles. I think I might have to go with Pundit on this one - 3 is best if you know they'll all show up every time, 4 is the compromise.

The low number comes about from the style I have as a GM, as in my sig. If you give the players and their characters complications rather than obstacles, that's best sorted out by a smaller group - 5 or 6 will never sort anything out because of the inverse relationship between group size and speed of decision-making.

I like small groups.  I've happily played with 1 GM plus 2 players... but the two players need to be pretty committed.  In an ideal situation, I'd have 1 GM and 3 players.  Again, you need all three of the players to be pretty committed.