I'm gonna be a burr in the saddle and say it's the original Ubisoft franchise!
Mega Man II was an early favorite of mine, the title screen tune was my favorite thing to come out of an NES. Then as endless sequels seemed to come out, I'd play them at my cousins and be disappointed. Mario 3 got so much better, what was Mega Man's problem?
Then we got an SNES at my house and Mega Man X was Everything. (Also the cartoon was coming out, and the hints of a dark collapse leading to the future of X tickled me) This was the evolution, the Real Sequel, Baby.....cut to five more installments that got worse with each release. (I haven't played X7, so I can't say.)
Obviously folks *do* like it, for me the problem is formula. The platforming is just okay (but the dash boots really improved that), and the shooting is no Contra but the weapons are fun. It's just too much repetition, the games are visually impressive, particularly where the bosses are concerned, but mechanically it's the same thing over and over, with the occasional inspired level design.
Nowadays, I kinda wish they'd do spinoffs with that great sprite work they do, like an SRPG with the Maverick Hunters from the X games. I was always dying to see how that organization looked day to day, but the plot of every game is them collapsing or turning on X and Zero, like the CIA or IMF in a Mission Impossible movie.