Another one I like a lot is my Pentacon auto 50mm 1.8 multi coating. If you can find one in good condition, this is a great looking lens you should be able to get below 50 bucks. At least that's the kind of price I remember last time I checked.

And it uses (like the Helios) the m42 screw mount instead of a proprietary bayonet. So that's a big plus for fans of open standards, even if it's absolutely irrelevant today, since you'll need adapters anyway.

But my favourite might be the Minolta Rokkor 50mm 1.4, that is just a nice lens. There's also a 1.2 variant, but it's harder to find and more expensive.