The question is - which stops (ND 0.6, ND 0.9.....) are the most frequently used? I am currently choosing Tiffen ones and since there are plenty of different stops to choose from and I have nearly no experience in using ND filters - your suggestions would be highly apreciated
There is no "most used". You use whatever is appropriate for the exposure conditions you have and the desired aperture and shutter speed (and ISO, though turning down ISO is almost always better than using an ND filter if it is an option). The more light you have, the more wide open the aperture and the slower the shutter speed, the heavier the weight of the ND filter you need.