I'm partial to the level 5-10 range where adventurers are powerful enough to feel heroic without having easy magical solutions for everything. Since I had to pick one number, I choose level 8 for pretty much the same reasons Jeffh gave above (good range of abilities without introducing the most problem prone magical elements -- best compromise).
Powerful magic is nice and all but the game gets a little dull when everything is solved by a spell.
Powerful magic is nice and all but the game gets a little dull when everything is solved by a spell.