Not only is it one of the most extreme examples of a good franchise turning to garbage, but it has the added poignancy of mirroring the state of Sega as a whole.
I disagree with bomberman, only because Bomberman Live was redemptive enough for me. I also disagree with Megaman, because I don't think that series has turned bad, just decidedly average. Kind of like Doom and Quake.

Honorable mention: Phantasy Star Online. Episode 1 was amazing. Episode 2 was alright. Episode 3 was terrible and I like to pretend Universe never happened.