I currently own the 90 APO-cron, 75 Lux, and 75 cron. I used to own the 75 summarit, 90 tele-elmarit, and a 90 elmarit.
So I guess I have some experience in ownership and useage. Here's my take:
90 elmarit (last version with telescoping hood.): Possible the best all 'rounder. Great deal, compact, great great IQ, sharp but forgiving, amazing, and possibly one of the most under-rated and underappreciated lenses in the lens lineup. By most accounts, it is optically and cosmetically superior to the 90 summarit that replaced it. I miss mine and, at times, consider returning to it. I list it here first, as I think it is "The Best Buy" of all Leica 75/90 lenses. Runs around $500-$800 used.
....however....
The 90-APO cron is pure magic. It's the sharpest lens in the M lineup, in terms of getting those micro details. This makes it great for compressed landscape work. It's a stop faster than the elmarit, but what you lose in return in size. It's much bigger, by feel, than the elmarit, but it's a no compromise lens. Plus, by Leica standards, it's a relative bargain to find one used (usually around $2K, got mine for $1700 last year)... CUrrently runs around $2K - $2500 used, $ 3500 new
The 75 summilux is magic, in terms of its drawing. It has a very different look than the modern 75's (summarit and cron, hence why I have the cron). It's a beast of a lens, so if size bothers you, don't get it. If you want the most unique signature (possibly as unique as the noctilux f/1), this is the lens to have. Lower contrast (macro contrast that is), and it blooms a bit wide open on the M8 and M9, but sharpens very much at narrower apertures. It's a beast of a lens, but great for portraits...actually, in my mind, the best Leica portrait lens. So if portraits are your thing, if you like the more classic Mandler style Leica look, AND size doesn't bother you, this is the lens to have. Runs around $2400-2800 used.
The 75 APO Summicron Asph is a terrific lens. It is the most modern in character (in line with Peter Karbe's designs), and it looks much like the 50 lux asph in character . It's much smaller in feel than the 75 lux and a better option for travel. I would not take the 75 lux on a long trip, but I would certainly bring along the 75 cron if my kit were a 24/35/75 kit. Many speak of this as the best 75 lens, and it probably is technically. It doesn't have the classic glow of the 75 lux, but it has its own modern look that i find very satisfying. It's why I purchased it, despite having the 75 lux. Runs around $3K used, $3400 new.
The 75 summarit is very close in character to the 75 cron, more modern in look, nice build, and a real bargain...This is the best bargain of the 75's, and if you like the modern signature, go for this lens....the only big drawback is the screw on hood, which really adds to the length of the hood, and the longer min focus distance (.9 meters compared to something like 0.75 meters for the 75 summicron). I owned this lens for a year when it first came out, but I found myself reaching for the 75 lux more. Great deal, though! Runs around $1,100 used, $1500 new.
The 90 Tele-Elmarit: I had the thin tele-elmarit. I liked it for its size, but not much else. I tried it out for about a month, and returned it to my dealer for a re-stocking fee. I thought it was a very technically sound lens. It's out of focus rendering was a bit harsh, a bit geometric, not as pleasing as the 90 elmarit. I already had the 90 APO-cron and thought I'd use the 90 Tele-elmarit in circumstances where size mattered. Turns out that I didn't care...I'd still bring the APO-cron, so the Tele-Elmarit left. It's a good lens, really, but just not for me. If size is your priority, this is the lens to get, as it is tiny. Runs around $500-700 used.
I have not tried the CV 75 color Heliar, but I have heard that this is a steal of a lens. Not as well built. Bokeh is pleasing, but not quite upto Leica standards, whatever that means (I think it's a bit of bunk...it seems to have a bit more geometry in the bokeh), and it's quite sharp, though not like the 90 elmarit or apo cron.
90 summarit: I have no experience, but I have heard it's a fair performer...