In Boise, it's probably 20. Default residential limit, and a lot of them are unposted, but generally you get one sign at the entrance to the neighborhood from the major street. So the 20 signs probably outnumber anything else. Beyond that, I'd say 30 MPH and 35 MPH are probably neck and neck, both being the common limits on arterial and collector streets. 35 is probably more common once you consider the farther west sprawl of Boise (with 30 MPH being more common around downtown, north end, the bench, etc.).
In Meridian, 25 is the default residential speed limit, so for the same reason as Boise, that's probably the most common limit sign. Because of that, I'm not aware of
any 20 MPH speed limits on public streets in Meridian city limits (not counting school zones); though, there are 20 MPH butting up against city limits (good job, well posted on each side):
GSV Link EDIT: I may have found a 20 in Meridian, though I am not actually positive which city it's in, Google Maps may be wrong:
Link (the top left of GSV says Boise, but if you look at the city limits on the map, it shows as in Meridian). After that, it would probably be 40 MPH, with 35 MPH close behind. The nicer/widened arterials are usually 40, and the narrower or more crowded ones (eg lots of driveways) are 35. Unlike Boise, 30 MPH is very rare (I can only think of a few, one of which used to be 25 until a few months ago).