The fourth gen was really the pinnacle of what 2D gaming had to offer. The fifth set precedents for 3D gaming: Nintendo with the analog stick, Sega with polygonal modeling, and Sony introducing dual sticks.
I think I'm gonna go with the fourth because of the DKC trilogy - I love those freakin' games.
"On my business card I am a corporate president. In my mind I am a game developer. But in my heart I am a gamer." - Satoru Iwata