I own/have owned:
15/4.5 LTM
21/4 M
50/1.5 LTM (sold it and replaced with...)
50/1.5 M
75/1.8
And I briefly owned a 50/2 collapsible, but it was manufactured incorrectly, in that it brought up the 35mm framelines, so I sent it back. I didn't really like the collapsing mechanism, but I've since gotten a Leica 50/2.8 from 1997 and I like it, so maybe my opinion on collapsing lenses changed.
My rankings would be:
1: 50/1.5 M
2: 75/1.8
3: 50/1.5 LTM
I really like the 21 and 15, but I don't use them all that often. The 50/1.5 LTM close focus limitation really bugged me, enough to get the 50/1.5 M, but I never had a problem with the results or the mechanical build.
I have a Leica 24/2.8, and I may get the 25/4 M simply because it's about half the volume of the Leica (with hood) and I could fit the 21 and 24 CVs in just about the same space as the 24/2.8. I won't sell the Leica 24/2.8, it's just too good of a lens. I already have Cosina's zoom finder. The 12 looks intriguing, but not unless i find an unexpected and large sack of money somewhere.