The boss GT series produce a reasonable wah sound, but don't expect the "real" guitar wah from the sixties/seventies.
If you are looking for that you are better off with a cry baby or a an old colour sound model.
Do you need it for a guitar or keyboard?
Fred
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Keyboards/Sound Units: Kurzweil 2600S, Roland VR-760, Acces Virus C, Roland G-800, Akai AX60, Minimoog, Machine Drum, Roland R8-M, mediastation x-76