It really depends what you want to do with the "noble."
As others have noted, fighter works well for knightly types. Given a human with a 10 int, that could buy:
1 rank of knowledge: Nobility and royalty (cross-class)
4 ranks of intimidate (good for staring down the peasants)
4 ranks of ride
1 rank of knowledge: local (cross class--you need to know your own laws)
It's not perfect, but it's not bad either. If you are willing to give extra class skills, you could add:
2 ranks of sense motive (skill ranks come from the extra points used to buy cross-class skills).
It's not the perfect knight, but it will get the job done.
If you go barbarian, you could add 4 ranks of survival and 2 more points of a cross-class skill (maybe diplomacy) or literacy and 1 point of a cross-class skill like speak language.
A ranger would also be a pretty decent choice. He could do everything the barbarian does and slap on some knowledge: geography and handle animal. (Since knights did a lot of hunting, the class is a perfectly reasonable choice. Multiclass to fighter later when you can afford fullplate).
There are, of course, plenty of other classes out there that people have suggested, and appropriately so, since there are as many kinds of nobles as there are people. A sneaky, treacherous noble might be well-modelled by the rogue class, for instance. Lots of skill points for social skills, longsword and chain shirt proficiency so he can look the part, and sneak attack for when he stabs you in the back. Paladin makes the righteous, grail knight type noble pretty well. Etc.
However, a few options that haven't yet been mentioned: from Player's Guide to Arcanis:
the Patrician class is good for a non-combatant type noble who mainly contributes via skills and minions.
The Gentry feat gives ride and diplomacy as class skills as well as a bonus to some other skills and the social priviledges of the aristocracy.