Between 8 and 11 depending on the character and the campaign.
Level 1 and 2 are kind of useless after a while. (I have started so many campaigns that it seems more to be an exercise in dancing softly) you don't really get in to your class until level 3 or 4. (Second feat and second level spells)
5 to 7 are fun since you can start seeing some of your character plans come to fruition. Also everyone can take a few hits and offer something to the combat.
After 12 is slowly becomes all or nothing. Everyone is now very powerful, but considering the amount of damage and Save or Die/Suck that can fly around, one bad roll can take you out completely, or end the encounter completely before the rest of the party gets to act.
In a good group everyone gets one combat a session to be the big hero. But sometimes it may be the same character doing the same trick for all the encounters.