If you want the most flexibility: Cleric. Hands down. You can make him into a fighter, an archer, a wizard, or a healer. And you can be good at at least two of those categories--maybe three--at once.
If you want the most interesting. . . well that's another question.
For interesting role-playing, I like playing paladins if I've got a good DM. It doesn't work if the DM doesn't like paladins, or doesn't appreciate the difference between interesting moral questions/choices and screwing the paladin with no-win situations though.
Fighters, rogues, and fighter rogues probably offer the most flexibility for making a unique but effective character. The multiclassed Fighter/Rogue/ex-Barbarian/Ranger/Paladin/Knight of the Middle Circle/Knight of the Chalice (or Fighter/Rogue/Barbarian/Ranger/Temple Raider/Holy Liberator) is probably the most mechanically interesting type of character due to all the options.
The multiclassed Fighter/Wizard/Spellsword is also very mechanically interesting because the concept is not well supported by the D&D ruleset so creating an effective character is a challenge.