Stadium is the best and most popular all the way, no doubt! I remembered spending hours bulding my own tracks backs days since it came out.
The Canyon version is a blast too Kristof, really like the grand canyon style & fact, really epic elements and stuff for building tracks. Was my first of the TM2 collection, tho. You'll find good servers out there.
Avoid the Valley version - it's pretty boring and has dull tracks and less participation, too. The vehicles are kinda slow and driving like boats.
No idea how it stands now, but I guess it doesn't changed since then.