6th
I was kind of undecided between 6th and 8th... If I have to pick one it'd be
6th.
Enough levels for interesting multiclassing, or even one level in a prestige class.
Enough hit points not to fear death from the first weapon attack.
Enough feats (3-4) to build some nice synergy or for a combat tactic combo.
Enough skill ranks (9) so that Take 10 actually covers most routine challenges.
Enough daily spells to handle more than just one fight
Several tactic options opened up by spells (invisibility for example).
No save or die spells yet.
No polymorph spells yet.
Not too many magic items per character, and not too powerful yet.
Characters have a history enough to be given important missions, but not yet over-the-top.
Of course lots of nice spells are still missing from the game, and another disadvantage is that metamagic is still very limited at this level. But overall I think 6th level is a very "safe" level for the DM to run adventures (not difficult issues to deal with), and good enough for players to have a character who is well-developed into something unique.