Phantasy Star Online. Soooo many nights were spent waiting on my parents to fall asleep so my friend and I could steal the phone lines to dial into SegaNET. PSO borrowed so heavily from Diablo and was a better game in my opinion.
Most games I have the fondest memories of were on Dreamcast: Jet Grind Radio, Soul Calibur, Marvel vs. Capcom (1/2), SNK vs. Capcom, Shenmue, Rez, Seaman, Skies of Arcadia, Grandia II, Tony Hawk 2-- the list could go on and on.