Gundam G-Saviour. God it's terrible. We fans have just written it out of our mental continuities, replacing it with Gaia Gear instead.
Lupin III: Green vs. Red is a good, interesting movie, but I refuse acknowledge it's (already dubious) canonicity. So in that respect, I pretend it never happened.
Devil May Cry 2. Never happened. Even Capcom tried to write it out of the canon.
Resident Evil: Code Veronica. A terrible game with terrible voice acting, terrible villains, and a plot that felt more like a parody of a Resident Evil game than the genuine article.
Also, come to think of it, RE: Operation Raccoon City and RE6. RE: Survivor still happened, though, because I have too much fun mocking that game's astounding lack of a save feature.
Season 5 of "The Wire" was just damn goofy, which is not a good state of affairs for a show that prides itself on realism.
The Tri-Star Godzilla, of course.
The Nintendo Wii. Nintnedo took an extended and inexplicable hiatus between the much-beloved Gamecube and the fun new WiiU, and would never jerk around their loyal fanbase with idiotic motion controls.
D&D 4th edition because... well... just look at it. All the classes play exactly the same and combat takes *ages* to resolve at mid to high levels. It's possible I'll have to pretend 5th edition never happened as well, jury's still out.
Any of Joshua Malina's scenes in "The West Wing". Season 4 onward simply had a massive uptick in the amount of scenes that featured Tobey muttering incoherently at an empty room.
...yeah, I think maybe I should be done now.