In BG1 Imoen is literally thrust into your party without any choice on your part, so she's a given. The next characters you meet are Xzar and Monty, and since you often need the help they are in. Then comes Khalid and Jaheira, Gorion's friends, and you are given and explicit quest to meet them. That's a full party in BG1.
By the time you meet Garrick, Kaigan (if you find him at all), Minsc, Edwin, or Branwen (and even Kivan, and Ajantis) you likely have a full party already, with some experience. Branwen, Garrick, and Kaigan are up for anything, but Minsc and Edwin want to go rescue/kill Dynaheir, while four characters already in your party want to solve the mystery in the Nashkell Mine. So they are a no-go atm.
Now I remember when I first started playing BG1, wandering around nooblike and clueless, the Harpers and Zhents in the party tended to fight while I was in the mines. Left to their own devices with no help from the PC, Khalid and Jaheira will almost always win the fight against Monty and Xzar. That's two slots free, one of which likely filled with Xan temporarily.
Back at Naskell, Minsc is the most likely character to fill out the remaining party slot. (Most people probably playing goodish/neutral characters.) That leaves us with CharName, Imoen, Khalid, Jaheira, Minsc, and Xan as the most likely party members in the Grand Generic Game.
The Xan slot seems kind of iffy though... I remember ditching Xan in Nashkell and taking on Branwen, before rescuing Dynaheir. Unfortunately, the logical person to go once Dyna is found is Branwen, assuming you want to retain Minsc.
After that most characters you will meet (like Alora, Eldoth, Faldorn, Yeslick, etc.) tend to be too far along in the game and the characters you met earlier are probably lagging in XP.
The most likely (imo) default party members in BG1 are CharName (ofc), Imoen, Khalid, Jaheira, Minsc, and Dynaheir. Obviously, that's what Bioware thought as well going into BG2.
In BG2 Imoen is once again a given, as she is the one to rescue you starting the game. Since you meet Jaheira and Minsc in the cell room as well, they too can be considered "default". That's four. Yoshimo is likely a welcome addition in most people's first games, so he's in. As the story unfolds, Yoshimo and Imoen can be considered to occupy the same spot in the party. So the character count is still four, leaving only two open spots.
I imagine most people will meet Aerie first (assuming they explore the promenade thoroughly), then Nalia (since she bum-rushes you in the Copper Coronet), and then Korgan and Anomen. Some other early (and easy) "gets" are Viconia and Jan. Edwin, Keldorn, and Haer'Dalis are a bit harder to get to. Cernd, Mazzy, and Valygar are outside of the city and so unlikely candidates for a default group also. I'll eliminate Keldorn since the storyline seems to suggest he retires with his family, and Haer'Dalis as he has too many strings attached in the way of quests and having to rescue him. That leaves me with Aerie, Korgan, Edwin, Nalia, Anomen, Viconia, and Jan.
Party composition is probably a concern for most people. CharName is (probably) most often some kind of fighter-type (as is Minsc, and even Jaheira), so we can exclude Korgan. Yoshimo of course is a thief, and Imoen can take care of everything the game requires as far as thieving skills go, so Jan can be safely left with his turnips unless he is filling a mage slot. In spite of Jaheira being a good healer, a "real" cleric would be a very nice thing to have. Additionally, though Imoen may be a mage, she is also in need of rescuing, so a mage of some sort is another good choice for one of the open party slots. The choices narrowed down: Aerie, Edwin, Nalia, Anomen, Viconia. Based on their class(es), we can put those characters into two groups: (clerics) Aerie, Anomen, Viconia and (mages) Aerie, Edwin, Nalia.
I'm pretty confident that the last two spots in the Grand Generic Standard Party are had by two of the above five characters. But which two characters?
My gut feeling tells me to drop Anomen and Nalia. I admit that Anomen annoyed me some when I first started playing (though he's grown on me some since then), and I've always actively liked Nalia. Based on reading a lot of different opinions online however, I get the feeling that those two get under the skin of a lot of folks. So goodbye, Nalia and Anomen. That leaves just Aerie, Viconia, and Edwin.
Edwin is no doubt very popular, and hands down the best mage in the game. However he is evil (somewhat important I think), a bit harder to get to in terms of plot, and there is a hint that he won't get along with Minsc (even though no real trouble comes of it). I'm tempted to drop him, but that would leave me with Aerie and Viconia, and that's divine magic overload. Rather than dropping one of those three, I'll instead pick a winner: which would be Aerie imnsho.
Aerie has a lot going for here, not the least of which is how early one finds her. She also, in one stroke, fills two party composition needs (arcane and divine magic respectively). She of course has a romance, a plus, even if Jaheira seems to be the designated winner. We now have a solid party with Aerie, and the rest of the group: CharName, Yoshimo/Imoen, Minsc, and Jaheira.
Instead of picking just one character for the last party slot, I'll leave it as an "open" slot. In fact, my first game was CharName, Yoshimo/Imoen, Minsc, Jaheira, Aerie, with the last being various characters I found along the way. I remember Korgan, Anomen, Viconia, and Haer'Dalis being in my party for a decent amount of time. Nalia and Keldorn had the spot the most however, but I let Keldorn go when it came time to chose if he should stay with his family or not. After that I kept Nalia all the way through the Underdark and beyond. I had planned on dropping her once Imoen was found, but that was not to be. By the time I got out of the Underdark, she had too many spells and experience to try out Mazzy, Valygar, and the rest.