I went with 4th but there are so many ways to look at it.
Let's see 1st could be good because the character hardly has a start, no real tales of adventure to tell yet, but then again the team will be starting from scratch after they probably put some real thought into their character.
Getting into 3rd and 4th level there has been time to have some adventures under their belt. The player is probable getting a pretty good feel for the character. Good level for the reality of bad dice rolls and overpowered encounters to hit. Far enough long to have more vested with the character, but not far enough long to throw the whole game off.
Lower mid-levels to Upper mid-levels, by now the character has lots of tales to tell. The player could be getting bored of the character by now, so less loss. Or they could be close to completing a major leg of the campaign and a TPK would be a major setback (or a great launching point for the right DM).
19th to 20th levels, hhmmm could be a good reason to not learn the epic rules. Or it could be the mountain collapses as the party destroys an artifact and they die as heroes. Or maybe they fell just short of the goal they worked so hard for.
So many ways to look at it.....