First character to go from 1st level to highest level.
I personally haven't had a character reach very high levels, again because I really haven't spent that much time on the other side of the DM screen. If I remember correctly, the highest-level character in our first long-running campaign was a fighter named Zeke, played by one of my brothers (a younger one than the first two I introduced to the game. I have five brothers and three sisters, and I've DMed for all but one of them at some point.)
Zeke reached somewhere around 27th or 28th level. We had transitioned over to BECMI D&D long before that point (not intentionally, as such - that was what was in print, and I still didn't realize there were any actual differences) so we were into Master level play. It was Monty Haul madness. Zeke and his cohorts had literally extra pages attached to their character sheets listing all the magic items they'd acquired. Even without all the magical goodies, BECMI characters at that level are pretty ungodly. Nothing short of a huge dragon was really much of a threat.
I had for a while been regretting being so liberal with the enchanted loot, and looking back with nostalgic fondness on the Caves of Chaos and the Isle of Dread. My younger siblings and cousins were pretty attached to "their guys," so I was reluctant to make them give up the characters. It was at that point that I suggested what would come to be known as a reboot: Let's retire this whole campaign, bust all these characters back down to level 1, and start over. I expected some resistance, or maybe outrage. To my surprise and relief, they accepted it, tentatively at first, and then with enthusiasm. The adventure began anew; the danger was real and the rewards mattered again.
As much as I generally prefer classic D&D over AD&D, I think the latter really had the right idea in capping character advancement around level 20.