Treviso impressed me the most, really. I didn’t knew anything about it, thus I wasn’t having any expectation. I ended up in Treviso simply because that’s where I’ve stayed on a trip to Venice (they have an airport and very good links to Venice by train). It was easier to set up base in Treviso and visit a few of Veneto’s cities.
To me it’s the perfectly sized city: not too big, not too small. Also, not very touristy (thanks to the closeness to Venice and Verona), but very close to tourist attractions so you can easily visit them.
The city itself is very beautiful, filled with canals (nicknamed Citta d’Acqua), lovely historic center entirely surrounded by fortified walls and canals (La Mura di Treviso).
Photos speak a thousand words they say.