Just use a high Cha character and put a decent number of skill points in diplomacy, bluff, and intimidate. That will give you the best options in 90% of the dialogues. Note that some dialogues are race or class dependent, so even taking a character with huge scores in non-physical stats and all the talking skills doesn't guarantee you every single dialogue option. I would avoid talking with paladins, despite their high Cha, because they turn down quest completion rewards (as do monks) and their paladin status can cut short many dialogues with evil characters.
I couldn't say, offhand, what the minimum required stats are for the estimated few dozen dialogues that can give you an XP advantage, and from the lack of responses I doubt anybody else can either.
You just gotta play...