6 because that's what Baldur's Gate, which really got me into RPGs uses.
No reason other than that. It's like liking x because the first y you did had x.

Also, I'm a pack rat, and with smaller party sizes I'm more often faced with having to throw away good unique items, because everyone is already using better ones.

EDIT: Also, of course this is just a general answer. Whatever works for a particular game is good for me.

Three; 2 offensive, 1 support is my favorite party ratio, and 6 characters is harder to keep track of, not just in gameplay but also narratively.

The only problem is when one character is markedly overpowered compared to the rest (I'm looking at you, FFF), in which you tend to switch to a 1 offensive, 2 support configuration that pretty much wrecks the game's balance; with party sizes of 4 or higher, there's enough weaklings in your party so you actually see some deaths every once in a while.

My own characters (as in Might and Magic): 4, 5 tops.

Recruitable characters with names and stories (as in Arcanum): as many as possible for the game to deliver.

I got low rated for saying one!? :P

WTF!?

I wasn't aware there were correct answers here. :P

-What's your favorite color?
-Blue.
-No, red. MINUS 1!

one.

I haven't played any Ultima games so I can't comment on that.
Eight would be too much imo, six is just right.

In computer games 4-5 including my character. That is, as long as we're talking about characters controlled at the same time "on mission" so to speak. It's nice to have a larger roster to swap some characters in and out for variety and special quest/mission needs.

In pen & paper RPGs I think 3 is best, 4 tops. Starting with 4 things slow down and the entire game gets more and more unwieldy and unfocused. 3 players + DM is perfect for keeping things rolling, keeping the DM in control of the game, and it's the perfect number to have the group in a sort of "balanced state of inbalance"- they most likely have enough various skills between their three characters to solve every problem, but not enough to be able to cover every angle and be ready for every surprise, so they have to keep being creative.

Four in general is a nice number.

I definitely like 3. Xenoblade Chronicles (not X), Radiant Historia, and many others have it. If it's so common it's got to be good.

In party based games that allow it I prefer one. I solo it and see how far I can get. Much more enjoyable for me and allows me to actually care about my character rather than care less about a whole group of characters.

I think six is a reasonable number, but I get so much anxiety by not being able to take characters because of all of the dialogue that I miss out on.

Three or four ^_^

Three that's the magic number......

Honestly i just fill all avaliable slots, second time thru one less and so on.

I would say 3 is solid