I have the last version of the Elmarit M, with the pull-out hood. It is compact, at about 80mm from the mounting flange to the filter ring. It is dense and a bit heavy however. There is minimal vignetting, and it has high contrast at all apertures. You adjust the aperture only to control the depth of field. Vignetting is gone at f/4. No distortion. It is a daily carry lens, residing in my bag along with the 35mm Summicron ASPH, 50mm Summicron (non-ASPH).
I have the Voigtlander 75mm f/2.5, which is really a beautiful, classic rendering lens. It is sharp, but not clinically so, and I find it useful indoors for portraits where the 90 may be too long.
I'm a bit of a 75-90mm lens fanatic. Here are my views on some others:
Elmar 9cm f/4 uncoated pre-war (1936). Very vintage feeling images with somewhat lower contrast. Sharp in the center wide open, but edges melt away. Sharp stopped down. Watch for haze. I had DAG clean and overhaul mine.
Great deal for $100, and you can use a 34mm screw in lens hood (bought on ebay) instead of some hard to find original Leica hood for this.
Voigtlander 90mm f/3.5 Apo-Lanthar. Mine came out of the box back focusing. I didn't realize it until after the warranty period. I sent it to DAG, who said that there was "no more adjustment room" for him to correct it. I discussed this with Stephen Gandy, who sent the lens back to Cosina Voigtlander in Japan who serviced it at no charge to me. Stephen has great customer service, I may add.
I think it may still back focus about 1cm, but I'm not going to complain at this point in time. The round trip to Japan and back to the US was several months. It may also be my anxiety when focusing the lens for the first test shots. I'll repeat the testing one of these days. Overall, if it's adjusted properly, it's a very fine lens. Build quality is not the same as the Leica, and there is more light fall-off wide open. No distortion, and not prone to flare either. Thin, small and light lens in physical dimensions.
Nikon 8.5mm f/2 PC in LTM. The classic David Douglas Duncan Nikon Sonnar. It's come back from an overhaul at DAG (clean the lens elements, get oil off aperture blades). So far, I haven't actually taken a picture with it, so I have no comments. However, there are plenty of good words about this lens online.
Mine came with a Nikon 8.5cm viewfinder, which was hazy. I took the elements apart and cleaned them, but in backlight I can still see some flare through the finder. The Canon 85mm viewfinder that I have is superior.
Canon 85mm f/1.9 in LTM. Chrome, and HEAVY. As with all my vintage lenses, mine was serviced by DAG upon receipt from the seller. This focuses sharply wide open and at minimum focus distance. Very nice rendition, with less contrast than the modern Leica Elmarit M. If it wasn't so heavy, and I wasn't so lazy I'd use it more often. Mine came with a very nice Canon 85mm viewfinder with parallax correction. This one is clear and doesn't flare in backlighting, unlike the Nikon VF that I have.
In terms of viewfinders (for LTM cameras), I also have the Leica SGVOO. It's nice, but the parallax correction is by a lower line in the frame, not by a distance dial on the finder. I like the Canon over this.
With an M4-2, you won't need any of the viewfinders, but I'm mentioning this for completeness.
Jupiter 9, 8.5cm f/2 aluminum body in LTM. Mine was fine at infinity. As you focused closer, the problem with the lens register appeared, and it was severely out of focus at minimum focus distance. Unless you like taking lenses apart and shimming them, I'd avoid this lens.
SLR:
Canon 85mm 1.2 L in new FD. Really a fantastic lens. Needs a lot of care at f/1.2, as the depth of field is razor thin.
Nikon 85mm f/1.4 AF-D. I think I would have preferred the AIS version, only because the manual focusing action is not as smoothly damped on the AF version. I don't own any AF Nikon cameras. However, there is nothing wrong with this lens' imaging ability.
Nikon 105 f/2.5 AIS. A classic Nikon lens. Designed to be flattering by leaving residual spherical aberration wide open. Stopped down, as sharp as can be. Long focus throw means that the lens needs accurate focusing. Even 50 feet is not infinity on this lens. My Leica Summicron 50 behaves the same way.