Eyeshield 21 comes to mind. Great manga, terrible anime with downright stupid changes.
I saw those changes, but they didn't kill the experience for me personally. I liked the animation in the important parts at least was good, the openings most of the voices and the way it was done in several parts, but many changes were pretty dumb, and comentators would kinda be annoying… specially the ending of the anime was painful.
But speaking about stupid crazy endings, what about claymore and FMA:.
Also people bashing on One piece: I have to agree that the animation comes and goes,hits and misses and all that... But it's still hit and miss for me, up to ennies lobby was pretty well done IMO. After that thriller bark was bad except the battle vs oars.
And for things after that, again it's nothing special or interesting except on moments. I don't remember liking something really after Bark, but many parts were okay.
Personally I dont have a vote.... really, there are some adaptations that are bad, and some endings or modifications that are AWFUL, but most of the time its bereable, or its just fine.
I have to agree though in the end Hellsing 2001 Was god painful awful.