The greatest two online gaming moments in my life:
Quake 1 co-op with my best friend Mike over our 28.8 modem (Direct Dialing, no less) going through scourge of armagon about 500 times.
BG2 Throne Of Bhaal: Beating Melissan and the Gorgon with three guys I met on gamespy. Went through the whole game TWICE with them.
Co-op has always been more meaningful for me. DM is ok, for awhile, but I prefer friendly interaction and teamwork. I used to think I was the only one, but apparently not.