My first one was a 50/2. Take your pick. If I had to do it all over again, knowing then what I did now, I'd go for a the CV 28/3.5 for a great option under $500. Heck its probably half that.
Depending on what focal lengths you like, somewhere in 28-50 is probably a good first lens, with an emphasis towards 35 or 50 unless you really know you like 28 mm.
For < $500, you can get a mint used Zeiss 50/2, a Zeiss 35/2 (if you look), a Leica 50/2 (the newest version if you look hard), a Leica 50/2.8, a multitude of CV lenses in the 28-50mm range, or a Konica 35 or 50. All of them are good lenses. If you want fast, the CV 40/1.4 and 35/1.4 lenses offer a lot of bang for the buck.
You can certainly find older Nikkor and Canon lenses for less than that too. While they are great lenses, they aren't modern and might not be to your taste. Some older Leica lenses should be lumped in there too.