I put in my vote for "3rd" because that is closer to the truth that even a vote of "other" would have been.
Each campaign starts where it should start based on what levels mean in a narrative sense; if we are starting the campaign as apprentices or the equivalent still learning the basics of their class we would start 1st level, if starting the campaign as people genuinely prepared for their first adventure - that kind of prove you can make it without your teacher's help moment - we start at 3rd or 4th level (I lean toward 4th, but my buddy that has just started his first campaign DMing 5th edition elected for a start at 3rd, and I started a campaign at 3rd before realizing I'd much rather start at 4th because it is the very last "apprentice tier" level).
I have an upcoming campaign that will start at 13th level because that is what best fits the narrative of the campaign being what some semi-retired adventures who have saved the world before.
I'll likely start the majority of campaigns that I run with 5th edition at 4th level for that "last apprentice level" narrative, and so that the majority of characters feel "feature complete" from the start of the game, rather than having to wait some number of levels until their "thing" is realized (i.e. the character concept of crack-shot archer not having to wait for the sharpshooter feat), but if I decide to run a campaign that involves a narrative of the uninitiated and under-prepared people forced to act I'll start at 1st level. I also really like the mechanical aspect, from the DM point of view, of starting with 4th level characters because it better fits my DMing style of not tailoring monsters encountered to the party's level and leaving it to the players to sort out when they are in over their heads, because each character is resilient enough to survive the player not being sure if they can handle a stand-up fight or not and trying it out for a round or two before deciding to flee.