Personally, 2nd or 3rd. I tend not to be a player very often, and when I am a player the campaigns tend not to start at 1st level.
Additionally, there was a very long spell (in fact, ever since 3.0e was published) when I never rolled higher than a '1' for hit points on levelling up* - this tended to lead to many replacement characters, who tended to start higher than level 1.
* Yes, really. My bad luck with dice is near-legendary in my group. On the plus side, I seem to live a fairly charmed life apart from dice (and cards), so I think it's a fair trade.
As a DM:
One party reached 15th level in a very-munchkin 2nd Edition campaign.
One party reached 18th level in my 3.5e "Shackled City" campaign. If we had bothered to level-up after the last session of the campaign, the survivors would have reached 19th level.
In my current 3.5e campaign the PCs have just hit 13th level. I expect them to reach 15th by the end of the campaign in a couple of months.
I don't expect ever to see another group reach those heights again, though - the current campaign has highlighted again the "sweet spot" of 3.5e, so I'll probably run another campaign at those levels; and it's looking increasingly likely that I'll give 5e a miss.