The "best" Trainz version of all time was undoubtedly TS2010 as it accumulated all the content and routes from previous versions, including compatability mode.
That said, things move on and last time I loaded up TS2010 to build a route it was surprising how aged it felt. Crude terrain textures which even at the most concentrated setting looked pixelated, garish looking track and roads floating off the ground surface and of course the ever present risk of splines becoming broken at interactive objects. TS2010 has also been off support for some time now, so no uploading content to the DLS.
Certainly for anyone coming into Trainz it has to be said that, providing you have the hardware, TANE is looking like the best option. With TS12 rapidly approaching life cycle end (and no sign of an extension from N3V) TANE will effectively be the only officially supported (PC) version. Many of the bugs in TS2010 persisted into TS12 with the added pain of the final versions having a very buggy and almost unusable CMP.
My perception of TANE is evolving especially now I have a laptop that can run it somewhat better in Surveyor. Yes there are considerable bugs remaining, the whole sorry history does no real credit to N3V, it still doesn't look anything like the target graphics and shouldn't need anything like the hardware actually needed to run. However the appeal of procedural track, no more broken splines, splines now sitting correctly on the ground, some of the new and excellent trees (the ST_RMM range for one) really starts to make it the first choice in the Trainz world.