Never flew on a Q400, but did fly on ATR42-300, ATR72-200, ATR72-500, Dash 8-Q300 and Fokker 50.
Of these aircraft, I found the most comfortable to be the ATR72-500. It is very quiet in the cabin, more like a regional jet.
Although the Q300 is quite new and modern, it is terrible for cabin comfort if the noise surpression system doesn´t work. The main problem here is the old-fashioned four bladed propellers, which make more noise than the six-bladed props of the newer ATR´s and the Fokker 50. The cockpit systems could use some updating as well. Compared to the older Fokker 50, the latter has a more advanced cockpit layout in many ways. For example the Q300 doesn´t have an automatic climb power button as on the Fokker, but the pilots have to adjust prop RPM manually. Generally the materials used on the F50 are of higher quality as well.