I've got all three of those lenses (and also the 25/4, 28/3.5, 35/2.5 and 50/2.5), and the lenses I find myself using most of the time are the 28, 21 and 50 (so much so that I may end up selling the 25 because I just don't use it). I love the 15, but it's too wide to use very often - there is a real danger of producing lots of "gimmicky" pics. And unlike some, I don't like the 15 for landscapes - there is a limit to the number of "something looking big in the foreground with the horizon as far away as the moon" type of shots I want to take (and with nothing prominent in the foreground, extreme wideangle landscapes just don't work for me).
The 75 is also a great lens - very sharp, if a little less contrasty than the others. But it doesn't get enough use - I much prefer wideangle lenses with RF and tend to use an SLR for longer lenses (My OM Zuiko 100/2.8 is always my first choice when I want a longer lens).
So I'd go for the 21, myself - it's a superb little lens, and is almost permanently attached to my Bessa R4A.
(But having said that, if I had an M8 I might think differently - the 15 on the M8 would give you near enough the equivalent of a full frame 21, and that would be a combination to be drooled over).