Orochimaru was casually tossed aside by a young Itachi; That was pretty bad.
Hidan had a decent death.
Sasori... I'm iffy on, he let himself die so he wasn't disgraced too badly.
Nagato had a bad ass battle JUST to get talk no jutsu'd. His death sucked.
Deidara had a cool death, but Sasuke hacked the manga right there and made it pointless.
Kakuzu was a bit cheated, but he was fighting how many shinobi and had his partner betray him (accidentally)? His death wasn't the worst.
Konan had a great death.
As did Kisame.
Same with Itachi.
It's probably Nagato or Deidara. Nagato's turn was such BS! This evil and emotionally, possibly mentally disturbed man who solo'd Amegakure. . .Was turned by a book.
And Deidara was cheated by Sasuke.. But I'll go with Nagato.