Depends on your preference, but here are the best US tracks out there:
Mcguirel UltraTrack V2k (available on DLS; best track I ever downloaded; so real looking and so many different varieties for different routes!)
STD Track (comes in High or Low poly versions; available at USLW)
Track Packs (available at USLW; comes in High or Low poly, grease or no grease, or squeal or no squeal)
Hope this helps!
Cheers,
Carfreak4