When I purchased my first film camera, I purchased a 50mm f2.0 prime lens to shoot with. While it was my first lens, I've since discovered, after shooting with many other lenses, that it was my favorite lens. Sadly, I lost it (along with my second camera body) in a classroom. The last few rolls of film I've shot were with a (borrowed) 28mm f2.8 lens, which is nice, but is only really good for wide shots.
In my mind, a 50mm prime is the perfect general-purpose lens. it's wide enough for landscape shots, but narrow enough for portraits. If I could only shoot with one lens for the rest of my life, it'd be a 50mm. With that in mind, I ordered a new 50mm f2.0 lens on ebay, for less than $20, from the same buyer. As before, it came in pristine shape, and takes amazing photos.
I forsee myself shooting mostly with this, possibly seconded by my 35-70 zoom. Hopefully before I leave, I'll do a write-up on my lens collection.