Aun'Va aside, i think Lucius the Eternal and Ahriman are both right up there.
Lucius can't win a fight against dedicated assault units to save his life, despite CC being his entire focus. His Doom Siren is a saving grace, but for just over his points you could take a 5-man Noise Marine squad with Rhino and Doom Siren champ.
Ahriman will lose in a 1v1 Psyker fight with just about any Psyker in the game, since Chaos has no psychic defence. Kinda sad when a 250 pt model can be taken out by a 150 pt Rune Priest without it breaking a sweat.