5/6 level is fun to play at, but I wouldn't like to be 6th level forever. I find it hard to make a world believable in which *no* NPC is above 6th level. Maybe that's just me. And I would hate to have my character going to be perpetually outshone by the NPCs. After 6th level you'd want to get to the good spells and the good prestige class abilities, and that takes a few levels.
So I said 13th level. I echo what people said about 12th level, but with 7th level spells available. If the PCs and (almost all) NPCs are capped at 13th level the campaign world will still have access to the most significant magic (and magic items), but there would still be some ancient mysteries hanging around (8th and 9th level spells). The PCs are powerful enough to handle boss monsters of up to CR 20 or so (with difficulty), and so nothing in the monster manual is really out of bounds. Most prestige classes have granted significant abilities. Many feat/class combos are possible, but most of the insanities of high level optimizations are still a bit out of reach. Political achievements are appropriate and believable, and provide a good alternative reward to xp.
There's a lot that people can do at this level of power, and you could play a long time without feeling the lack of advancement.