I would have to ask a couple questions first
1. What are you shooting?
2. What are you shooting with?
3. Do you like clinical sharpness or vintage character?
I shoot BW film with an M2 and chose my lenses specifically to give a vintage feel (something I was trying to emulate in digital post processing from my canon gear). So keep in mind, my experience comes solely from that perspective. Also, I use film as a daily carry, not landscape specific or portrait specific. For that type of work I use my canon and some L glass.
Summaron 2.8 - This is my most used 35mm. I think it gives a lot of versatility and I LOVE the way the lens draws. If I was to keep just ONE 35mm, this would be it IF I didn't plan to do any night or lowlight work a lot. 2.8 with film pushed to 1600 still isn't fast enough a lot of times. If your camera can handle 6400+ ISO, maybe go for it. All in all it's my favorite lens on film followed closely by the 50mm Rigid cron
Nokton SC - this lens allowed me to go into a concert at night and shoot without a press pass and pushing to 1600. It didn't look like a "pro" camera, so I was allowed in. Had I not had this lens, I wouldn't have gotten to shoot. I've had good results from this lens and it's about the size of the Summaron. It has its critics, but for the price IF you are planning night shots, or just a cheap, non-critical lens....it's hard to beat the value.
Elmar 3.5 - mine is a 1938 I think. I got this old dog at an estate sale and I think it's a lens that is very specific to a classic look. Its totally uncoated and WILL NOT take filters without an attachment. Also, the aperture control is not intuitive (for me anyway). Where it's fun? Where I'm gonna want some flaring, and can walk around with a set apertureand not move it much. Maybe some creative street stuff? This thing is TINY though. I meant it's about as small as a lens cap when mounted.
Hope this helps! Most important thing to do is go take pictures. So if you see a good deal, just get it and make good compositions and exposures.