Mine's Katekyo Hitman Reborn because:
.The ending felt way too forced (probably because it's serialization was ending).
.We didn't see more of Kawahira and the species he's apparently by a random chance part of.
.In the end, Tsuna is still the same whiny, weak kid from the beginning of the series.
.Reborn left Tsuna for a while after giving up on pushing Tsuna into become the 10th generation boss, but then comes back with the title "Neo Vongola Primo" which sounds ridiculous.
.The ending just completely insulted the character Xanxus who wanted to become the 10th generation boss unlike Tsuna who constantly rejected it.
.The ending left many unanswered questions
So what are your "favorite" worst endings guys?