Assuming I would know nothing about the adventure before making my character, it would depend a lot on the system, and the class I was going to play.
With 3.x, I guess I would choose 6th. 3 levels of spells for all casters, and an extra attack for melee focused folk. You can make +2 items, and have one level of a prestige class (and many of them are front-loaded) Also, hitting DC 15 skill difficulties is pretty easy, and even DC 25 or so is possible, if unlikely.
For 4E, I would tend to go with the same level. A couple of dailies and utilities, plus some encounters gives a good amount of versatility. A bit more staying power with the +1d6 to healing word-type powers and
a good amount of magic items can be created, though not +2 ones.
For 2E and 1E, I just do not remember enough of how to play them to make a real choice.
BTW, I am dying to try E6 someday, if I can talk my group into doing it.