I haven't bought FD in ages so I don't know the used prices but the 28mm is probably still the cheapest option if price is a consideration. The 24mm is the best one though.

The 20mm f/2.8 is kind of soft especially wide open but I still like it a lot. Got lots of great shots with it when I really needed the ultra wide.

The 24mm f/2 is a wonderful lens, so sharp even in 11x14" prints.

The 28mm f/2 is alright but for some reason I don't use it that often, I usually go for either the 24mm f/2 or the 35mm f/2. The 35mm f/2 is another amazing lens, so sharp the detail just calls to be printed large.

I long lusted for but never owned the FD 17mm f/4 but if the similar looking 17-40mm EF optics are anything to go by then I wasn't missing much. Stopped down my EF 17-40 does alright but wide open the corners are really squishy. The 20mm seems wide enough anyways most of the time but one day if I see one for cheap I may need to try the 17 f/4.