Overlook SX - Halo: Reach - Purpledinosaur0
To self-elect - and to disobey direct orders from Cassel - I think Overlook SX, although going through quite a bit of variation, provided some great SX racing for its time.
Whitewater National - Halo 4 - King Edward I
This track was late in the lifespan of Halo 4 but reinvigorated a lot of people's interest in competitive racing at the time. I remember a lobby of 10 or so people racing on this map competitively for no specific reason because it was so enjoyable. As an aside, this track won overall in Best of Halo 4.
Asimov Beach/Stadium - Halo: Reach - Jeffh3962
This track may not be remembered well, but it was forged on the map Tempest and took advantage of it with a really distinct endurocross section with logs and grass. It's one of my personal favorites. It also was very technical and featured 3 or so split rhythms.
Special Mention: A.51 Motocross - Halo 2 Anniv. - MtnDew97
This track was very ambitious when being forged and I doubted its completion while watching him forge it, but it did get done. It boasted at least a minute long lap time (as far as I remember) and raced very competitively with a unique approach to the placement of terrain