Sonic without a doubt. I grew up with Sonic. Sonic Adventure was my first videogame ever, and later I played Sonic Shuffle (which I honestly enjoyed more than Mario Party), Sonic Heroes and from there, many of the games in the series. I've played almost all the games from the classic era, I've played the Sonice Advance trilogy, Sonic Mania, and many others. Sonic is the videogame franchise I grew up with, and the one I've played for the majority of my life.
Not to say I don't like the Mega Man franchise. In fact, Mega Man is a strong candidate to be my second favorite videogame franchise ever.