I assume you're asking about 5e...
Probably 1st. Balance is mainly in the spotlight mode and DM-managed. You actively keep the flow of the game moving and make sure everyone gets to participate, rather than expect the system to do it for you.
Personally, I have not found any level of 5e entertaining as a player. DMing's never boring, though.
10th is the statistic that WotC came up with (and, eventually, shared) when they were designing 3e. But, that's really a question that encompasses much more than 5e (afterall, campaigns can transition across editions). My old AD&D campaign went past level 14 (the lowest level characters never exceed 14th, one made it to 18th), but was so heavily modified it was barely D&D at all. 3e campaigns I was in started in 3.0 and went the entire run, wrapping at 13th level (and, wow, were the cracks showing), the one I ran only went 6 levels or so, but it was a defined story-arc. 4e campaigns I've been in or run have gone to Paragon, one of those is on hiatus and going to start Epic when it returns, and the one that I'm still running just hit 18th. I've yet to run what I'd consider a 5e campaign: I've been running Encounters, those re-set at 4th, and intro games at conventions which are one-shots by nature.