Original Xbox. The Dreamcast really didn't bring anything new to the table and lets be honest, it was kinda a dead system before it even launched. THE PS2 hype and launch eventually killed the DC and hype for the system super fast. Yeah it had a lot of good games, but none of them really caught on with the public. But Xbox really brought a lot of innovation and went into areas that both Sony and Nintendo were not willing to venture into at the time.
* Console Lan (Halo Lan Parties)
* Xbox Live - This set the foundation for what we are used to now. It's funny that years later Nintendo still doesn't have the same setup that Xbox had when it launched live. If there was no Xbox live would Nintendo even have an online service now?
* Popularized dual analog shooters.
* Xbox also brought in a lot of older people (especially in with the PC crowd) to start console gaming.