Wow, very hard to answer.
Genesis gave me Sonic, Shinobi, Streets of Rage, Golden Axe...
Saturn gave me arcade-perfect Vampire Savior, X-Men vs SF, MSH vs SF...
Dreamcast gave me Soulcalibur, Crazy Taxi...
I might have to go with the Saturn. Arcade-perfect conversions were a rare thing at the time, and discovering import games and the 4MB cart led to hours of enjoyment with the aforementioned fighting games, especially Vampire Savior.