Crenshinibon has most things covered. Personally I prefer a party of four, once I get more than that in a party I start to lose sight of things, I guess I have a mental deficiency which impairs my ability to keep track of what 5 or 6 people are doing.
If you're playing an un-modded game you are almost forced to have a thief of some sort in the party*, like Crenshinibon, I prefer Jan. Nalia, is a viable alternative, as is of course Yoshimo, but you probably know what happens with him. Which can make choices easier if you want to have Imoen in your party after Spellhold of course.
Another factor which will affect who you take with you is your attitude towards alignment. If you want a 'good' party then obviously Edwin and Viconia are out of the question, unless you want a romance with Vici with an eye to saving her soul of course. If being good matters, then Aerie is an excellent choice, I'd disagree with Cren on this and say IMO she is potentially the most powerful NPC, but each to their own.
If alignment doesn't matter, then Edwin and Vici are the most powerful of their type as has been stated, and for my money, the best tank is Koran, who also has great dialog. Bear in mind that Minsc is supposed to be good, and can get stroppy with Edwin, and may even attack Viconia.
The main deciding factor is of course your main character, once you decide what you're going to do with her/him, then giving further advice will be simpler.
* If you use the G3 Tweak Pack there is an option to disable all locks and traps in the game, thus negating the need for a thief if you so wish.
[QUOTE=Darth Gavinius;1096098]Distrbution of games, is becoming a little like Democracy (all about money and control) - in the end choice is an illusion and you have to choose your lesser evil.
And everything is hidden in the fine print.[/QUOTE]