Post Audio Media has what we consider superior Accordion samples in terms of expression and versatility: Accordion demo
This demo was recorded life with a patch that uses the mod wheel for filter and amplitude control and several general purpose controller faders for crossfading between the different timbres.
....This one has a lot of "dirt" & character and is great value too.
I find the eduardo tarilonte accordions for Kontakt (Best Service) very good indeed, hi quality, there is a nice bandoneon in there. For Tango, I took the "Play The Tango" CD (Big Fish Audio) and created my own instrument for exs24, took some work but it sounds terrific.
Generally an "accordion" sample is not the same as a Bandoneon sample so watch out for that-they sound quite different.