If you target a creature, you make one roll, and starting with the most powerful active spell, you use your result against the DC (11 + spell caster's level) to see if it's dispelled. If not, you go to the next lower ability and see if the number you rolled would beat that. And so on until an effect is dispelled or you run out of effects. Once an effect is dispelled, the dispel ends.
Wouldn't the effects normally all be at the same caster level? Even if some buffs were from the 6th level cleric and others from the 7th level wizard, it seems if you fail to dispel one you'll likely fail against all the rest.
edit: Maybe I should start a different thread. This seems too specific a discussion for the current topic.