Original Xbox >>>>>> Dreamcast
MS redefined gaming with the original Xbox. Games like Halo, TES Morrowind, Kotor and Fable would never have come to consoles without Xbox. The games I mentioned were far better than pretty much anything the Dreamcast offered (except Shenmue). I have always seen Dreamcast as an overrated platform. It's mostly remembered by the sad death it had and by representing the end of an era in gaming.
Disagree on the overrated part although I see Xbox as the better platform DC is remembered for being ahead of its time that's why its death was so significant in gaming, a case point is how Xbox is seen as a spiritual successor to DC as it did what the latter did only better starting with the more cleaner online set up. On software DC lacks mainly in quantity but not really quality for example DC had the best versions of 2D fighters like MVC, MVC2, SF3 Third Strike, CVS, CVS2 and even on the 3D fighter front it had DOA2, Soul Calibur, Ready 2 Rumble and VF3 other case points in the library no platform had games like PSO, Jet Set Radio, Power Stone, Shenmue etc...
Another thing is some of Xbox's traits and even a few IPs were derived from the DC for example we mentioned online which Xbox executed well and pushed with the likes of Halo 2 but don't forget DC had Quake 3 online with cross platform support with PC before that and MSR is not only the precursor to Project Gotham it's the same team.