RangerWickett said:
I'm tempted to say 9th, for the same reason.
Ditto. On 9th, you're still away from most of the high-level magic, your non-caster classes still shine, and casters have enough spells to prevent the annoying "I've run out of spells - we'll rest!" stop-and-go.
With one (or two with maxed out Int) 5th-level spells, preparing
Teleport will massively hobble your casting... and people get access to one of the most interesting spells -
Telekinesis and additionally, 5th level spells allow clerics to
Raise Dead occasionally... a godsend.
I.e. on that level the casters get a taste of "real power", but still do not outshine the lasting power and impact of the non-casting classes (and on 9th, everyone gets another feat, hence the fighting/rogue-ish archetype get into the region, where they've got enough feats to work out a character concept completely.
Furthermore, on 9th, it is possible that you've got one or two levels of a PrC, so you'll have "your" character, able to do everything he should be able to do.