Mandarin. unlike most others he was done with the intention of screwing the character over and turing him inot a joke
Next would be Jarvis
He went from a loyal friend and butler in the comics to a computer A.I in the movies.
Those imo are the 2 most comic inaccurate adaptions with Deadpool bringing up the rear.
As for poor movies, all studios have had them and are varying degrees of poor
As to casting big name actors who don't fit the role but are used because they draw audiences (Jessica alba, Ben Affleck ect...), i've noticed this more and more recently. Glenn close as Nova Prime, Bencio Del Toro as the Collector, James Spader as Ultron, Jamie Foxx as Electro. None of these fit the characters imo, even though they are good actors and will do a good job.
Magneto: Does it ever wake you in the middle of the night? The feeling that one day they will pass that foolish law or one just like it, and come for you? And your children?
Professor Xavier: It does, indeed.
Magneto: What do you do, when you wake up to that?
Professor Xavier: I feel a great swell of pity for the poor fool who comes to that school... looking for trouble.