What series had horrible endings?

Depends on lvl of what you call horrible:
1. AOT I mean nothing could even say good word about it. It looks like cutted and rashed
1. GOT - it's better then AOT and to the people who didn't read the books could work;
2. Bleach - rashed in the end;
3. Naruto - this is example when you try combine everything in one arc, and can't chose one ending;

For example, in comparison decent ending:
1. Inuyasha - ending was good enough, the only thing that not turn it into great was fact that no one died in the end except villains;
2. Rave Master - could be better if was not rashed by Jump editors, so last fight was cutted in half, but was still good;
3. Death Note - sorry fans but for it was not great ending, good yes...maybe;
4. Hunter x Hunter - ending was good, but not the best...healed from any wound God ruined it;
Great Ending:
1. FMA - well, it is obvious that I put it here case it have sence and story was leading there;
2. Code Giass season 2
3. Gurren Laggan - just beautiful
Berserk anime
Pandora Hearts manga

Out of league:
FairyTail and other shitty anime or manga.
You mean the anime? The series has not ended tho. New arc did start but then came the loong hiatus.
Yes, I meaned anime...also I read manga, and I think that Togashi ruined everything with intro those 13 princes. Too many characters to deal with made things instead of going crazy, it wend with very slow motion.
So I was talking about anime only...it has ending decent one, but not great one.