KevinH put together this long list of games and asked people's opinions on which number of players were optimum for each game.
A good idea, but for me very unwieldly to go through the process of adding a comment to a geeklist for each and every game. So I'm going to make a mad dash through his list of games on my own terms.
I'll leave out a lot of the games on his list because I haven't played them or don't know them well enough to have formed an opinion on the question of number of players.
Puerto Rico - Best with five, but still quite playable for four or three.
Tigris & Euphrates - Best with three, then four. Playing with two was okay but didn't bring me back for more.
Power Grid - Rather surprisingly, works even with six, and works even with three. Four might be best. The auction is more interesting with more players, but you wait longer for your turn.
Princes of Florence - Five is fine if you've got the time, but the game works just as well with four or three.
Ra - I like any number from three to five.
Settlers of Catan - Three or four, either one. The board's more crowded with four, but you make adjustments.
Modern Art - The more the merrier! But the lower numbers are fine, too, if the players already know the game.
Taj Mahal - Five is probably best, but I've burnt out on this one.
Samurai - I think four is best, but three is good, too. Two is a different game.
Ticket to Ride - The more the merrier.
Amun-Re - Works very well for any number from three to five, though there are slight differences that a smart player will take into account.
San Juan - Three or four.
Through the Desert - This works all right with two, but I think it gets more entertaining as more players are added.
Acquire - Five is probably ideal, but it'll handle three to six well enough, and even two if both players control two hands.
Carcassonne - Good game with any number. It's just... how long can you stand to wait for your turn?
Maharaja: Palace Building in India - The more the merrier.
Traumfabrik - The more the merrier.
Union Pacific - Actually gets a little better as you progress downward from six players to three, but six is fun anyway. (Lately we've tried a partnership version that we really like. Your partner is the player opposite you in a six-player game, and you keep your declared stock together.)
Blokus - Four, obviously.
Kardinal & Konig - Three is excellent, four is really good, five is a bit dicey.
RoboRally - Any number from two to eight works fine if you actually like to play the boards and aren't just in it for the option card mayhem.
Thurn and Taxis - Two, three or four all work fine.
For Sale - A little more interesting with more players, but only a little. The real key is to get the original edition.
St. Petersburg - Two, three or four all work fine.
Medici - Five is probably ideal, yet the game usually gets played because you're looking for something to handle six.
Time's Up - 8-12
Attika - I like more players so that the "connection" win gets gamed.
Vinci - I'd say three or four, as downtime gets to be a big problem.
Domaine - Haven't tried this with two. Three and four are fine.
Bohnanza - Gets a little thin at six, even thinner at seven. No problems at three to five.
Lowenherz - Really needs four.
Scepter of Zavandor - Worked okay the two times I played, once with four and once with three. Patience becomes an issue.
Liberte - This CAN work with six, but finding six people who all like it and will put up with that much luck of the draw may be a challenge. Easier to put your skills to work with fewer players, and works fine all the way down to three.
Blue Moon City - I've only played it with four, where it works fine.
Alhambra - Thin but playable at six.
Mu and Mehr - Why would you play this except that you had five people who wanted a trick-taking game?
Hansa - I've heard this is good with two, but I've only played it with three or four, liking it fine at either number.
Stephenson's Rocket - Seems like two should work, but I only know the game for three or four. Four is a little more interesting, not a lot.
Daytona 500 - Three is ideal because you get so few cards with four, but four is lots of fun anyway. Also works amazingly well for two.
Diplomacy - The only real Diplomacy game has seven players.
Royal Turf - Two isn't worth the trouble. Three is fine, but it gets better and better as you work your way up to six.
Showmanager - Best with five or six.
Kremlin - The more the merrier.
Java - I've heard two will work; I know three or four will.
Mexica - I like four. Not sure I've tried it with any other number.
Ave Caesar - Why run this with fewer than six?
Mystery Rummy: Jack the Ripper - Conventional wisdom says this is ideally a two-player game, but I also like it with three.
Ta Yu - I like the partnership game for four the best, but a two-player game is like a partnership game where you don't have to wonder what your partner is thinking! I've never been tempted to try the three-handed version, but I do like the solitaire very much.
I'm the Boss - The more the merrier.
Can't Stop - Two, three, or four.
Carolus Magnus - I like three best, but the partnership game has its own peculiar merits.
Siedler von Nurnberg - Four is better, but three is fine.
Um Reifenbreite - Needs all four.
Sticheln - Three is best. Gets worse for every added player.
Cribbage - Two-handed is the classic game. Can be played with three or four, but with nowhere near as much opportunity for cleverness.
Loopin' Louie - Obviously needs four.
Big City - Three seems ideal, four good enough, and five too many. I'm told it words fine for two, too, but haven't tried it yet myself.
Himalaya - The more the merrier.
Elfenland - The more the merrier, but does work at every number, even just for two.
Kreta - I like four a little better than three.
Hey, what about Breaking Away? You want four at a bare minimum, really five, and can take six or seven with ease, eight if they all know what they're doing.
Thanks! Glad I got that out of my system!