As a DM, I prefer to start a long campaign at 3rd level; as a player, at 5th or 6th. I hate starting at 1st level and being a 'newbie adventurer fresh from the farm/palace/department store clerk job' - I like my "adventurers" to have a pre-adventuring career that actually gave them some skills.
Honestly, my preference for D&D would probably be to start at 5th level and only ever gain feats (and, I suppose, new spells of the existing levels), maybe gaining an actual character level every three or four months of real time, capping at 10th.
6th is probably my favorite level to play at - 3rd level spells for all but no 4th level, 3 character level feats, advanced or easy-to-get-into prestige classes, enough past experience to be a believable knight, veteran soldier, special ops, etc., and enough levels to really have the character fit the concept.
I'll say 6th.