Madden as a franchise has probably given me the most enjoyment, because I've played it through so many seasons. There's no other sports game I've played through as many generations.
Still, no individual Madden game stands out to me the "best sports game", so it's more of a longevity of a franchise thing than individual greatness of any one game.
The game I thought was really ahead of it's time was Bases Loaded for the NES. The create a player and build a team and train your players to build up their abilities aspect of the game was really incredible for it's time. I played he hell out of that.
NBA Jam was my favorite sports arcade game.
NBA Live 95 was out my freshman year of college, and we played four player NBA live every day for hours, so I probably have better memories of that than any other sports game even though I never really could get into any other NBA live game put out.
I loved NBA Street 2, and played that to death when it was out as well. I guess I'd still have to give Madden the best franchise. It's really hard to go in any other direction. Most of the other games I loved were for personal reasons more so than that they were greater franchises.