OG Xbox is basically a Dreamcast 2. So much Sega goodness.
For me, its a permanent Outrun 2 player.
You cant go wrong with Jet Set Future, Orta, Otogi 1+2, DaiSenryaku VII, Fatal Frame I+II, the Dead Or Alive offerings, Phantom Dust, and the original (and best) Red Dead Revolver (not Redemption).
Back in 2001-2002, I was an xbox doubter and hater - until I saw how many Capcom and SNK fighting games were on the system - and they were online. A first for consoles. It was a dream come true playing Street Fighter III + Capcom vs SNK 2 online back then. My mindset back then did not think 2D Capcom fighting games were gonna be on an American made console, and online no less.
There's a company out there that makes a high quality HDMI adapter for the system - no modding needed. Chimeric Systems.