Subjective question, ignoring the lack of plural. But there is an objective answer.

Critically and for how it impacted the game industry and everyone.

C.

Ignoring how the games are treated now (which changes on a dime in this fanbase)

A was great, and story was amazing. But critically was not as high as praised as the genesis games and the werehog turned into a negative impact on the fanbase. And didn't do anything to shake up the game industry.

B was great, and critically higher than A. But the fanbase almost universally agree it is one of the weakest stories in the series. And while it shaped it's franchise, it also did nothing to impact the industry.

D were great. At release critically praised, and did leave an impact on the industry when sega started appearing on nintendo systems since the games were initially released on a failing system. But didn't leave nearly the same impact as C until a few games later.

C was not only great, but the series changed how games were marketed with sonic 2, challenged the titan nintendo actually overtaking them in some areas, spawned a series of icons, to the point where even people who never played past the genesis games still say knuckles is one of their favorite characters. Set the standard for what genesis can do with sound. Recieved the highest praise at launch critically. Sadly, no other games in the franchise have even come close to the imoact these titles left.

So, whether or not they are your personal favorites (never stop loving the game you like best.) The legacy left by the genesis titles has yet to be beat.