SEGA GENESIS R.I.P. 1989-1995 Units Sold 29 million WW
NINTENDO 64 R.I.P. 1996-2000 Units Sold 32 million WW
The Genesis , as every system not named Nintendo used to fare in Japan, made squat (in Japan) during the long Nintendo reign, but it battled neck to neck against it in Europe where its predeccessor the Master System was very popular, and in America where it gained the hearts of many players specially with their exclusive and revolutionary sports games (at first EA sports only made games for Genesis, and Sega also had their strong first-party line-up). It was Sega's Golden era. In 1994 the Genesis outsold SNES in America. The Genesis was the craddle to many of Sega's popular franchises and many third-party exclusives, Joe Montana Football, NFL Sportstalk football, World Series Baseball, Sonic, Golden Axe, Streets of Rage, Guardian Heroes, Phantasy Star, Shinobi, Thunder Force, Virtua Racing, Shining Force, Beyond, Oasis, WonderBoy, Strider Hiryu, Splatter House, Castlevania Bloodlines, Contra Hard Corps. The Genesis also had the most famous fightning franchises of the era in Mortal Kombat and Street Fighter, and other third party gems like Earthworm Jim and Road Rash. However, it was Sega's trademark to renew themselves and create original, fresh-concept games (like they used to do until Dreamcast at least) like VectorMan and Comix Zone.
OT: The N64 had all-time gems like Mario 64 and Zelda Ocarina of Time, but if I had to choose between one library and the other right now, I'd take my Genesis without question, so many excelent games in different genres make it the winner for me.