If you want a fast & super tiny lens then this Pentax M 40mm f/2.8 SMC easily fits into your pocket and is fun to use. Image quality isn’t amazing but it has very little distortion and almost no ghosting.
This Pentax Super Takumar 105mm f/2.8 is a decent lens stopped down a wee bit and you can’t go wrong with it if you find a good deal. Being a M42 mount you can use this lens on some Digital camera bodies with an Ebay M42 Adapter. Also split focus screens from Ebay make focusing a lot easier!
There are some extremely amazing lenses out there and this Pentax-A 100mm f/2.8 SMC Macro is a prime example of mechanical and optical perfection. However this is a hard to find lens and still costs just as much as a newer auto focus 100mm macro lens even today. For perfect focus I would check Ebay to see if you can get a split focus screen for your body.
The Pentax Super Takumar 50mm f/1.4 Asahi is another amazing example of a really well built M42 Manual Focus lens which Pentax is famous for. If you don’t mind manual focus then this little lens is great for even today’s digital cameras with M42 adapters.