I am not a big user of 90's anymore. Used to have stacks of them, including the SOOZI 90f2 - never needed an exercise clas with this lens - it is HEAVY!
I do still have some left, the Apo-Lanthar 90f3.5 from VC and a couple of 90f4 Elmars (collapsible version) and probably some more in the camera cabinet.
It all depends what you are going to be using it for, low light and "soft" portraiture - the older Big Bertha 90f2 Summicron is actually a very good lens. The later version of this lens (1980's) was not as good in my opinion and then came the 90f2 Apo-Asph. Markedly better and bigger, but somehow I never got along with it.
The Elmarit M 90f2.8 is a good, lightweight lens, soft enough at 2.8 to be useful for portraits, but very sharp stopped down. The earlier version, the Elmarit 90f2.8 is almost as good, but a bit prone to flare and "fungus" between the elments. The light weight Tele Elmarits 90f2.8 (small and compact) but these you have to check as they tend to have serious fungus between the elements and it is a bit soft wide open. However, it is small and compact and if you find a good one, they work well.
The C Elmar is somewhat of an unsung hero. Very good performer and very compact. Check that your camera and the lens is matched for focus though. It was made for the Leica CL and the focussing cam on the CL and the rest of the M's has a slightly different curve, particularly in close focus. However, it is a f4 lens and that gives it some latitude for focus shift.
