If we go through the biggest L's by character:
Gohan - The whole series built him up to be THAT dude, but after the Cell games, Gohan became straight flabby and sick

Vegeta - Either Android 18 or getting humbled by Perfect Cell after letting him go perfect. The loss to Cell is embarrassing because he could have scored the win for once, but there's something more inherently disrespectful about getting his arm broken 18 since she wound up giving it up to Krillin of all people right after the fact. You know Vegeta must have felt a certain way about that moment every time he saw 18 and Krillin at all the Z-fighter family gatherings.
Krillin - Fangs the vampire has to be his most embarrassing L in this case. Krillin went into the fight too full of himself and didn't even land a single punch... even Yamcha got a win in Baba's tournament (albeit with Krillin's help... but STILL.) Getting blown up by Frieza has to be up there, but to be fair, Frieza was killing everybody on Namek.
Yamcha - The Saibamen, hands down. There's also the whole scene where he got impaled by Dr. Gero... and an honorable mention goes to his failed attempt to train at 300x gravity in the anime (which spawned a classic thread).
Tien - Either getting his arm ripped off by Nappa or getting choked out like Melvin in Baby Boy when he tried to fight the Androids.
Krillin took the most L's of any Z-fighter IMO, but at least eventually won in the end. On that note, I would say that
Vegeta, Gohan, and Yamcha took the biggest L's in the series.
Vegeta is Saiyan royalty, but will eternally be stuck in Goatku's shadow, and his soul will burn slow forevermore since he knows it.
Gohan's whole existence peaked during the Cell games when he was 11 years old.
Yamcha is Yamcha, no further explanation needed.
Krillin took his lumps but ended the series as (arguably) the strongest pureblooded human on the planet, and wifed the coldest woman in the entire series. Nothing that happened to Krillin was more embarrassing than Yamcha's loss to a Saibaman, losing his girl to Vegeta, or Vegeta getting his arm broken by 18, let alone Gohan becoming soft after the Cell games.