I don't know if there's a "best" lens, it depends on the job I guess. My always-on portrait lens at the moment is a russian Helios 44, famous for it's swirly bokeh effect. There are many variations of it, the earlier ones somewhat creamier with more aperture blades, the later ones sharper but with lesser blades.
Otherwise I used to try out many cheaper Pentax K lenses, some of them also available for M42. Some brands like Revue, Ricoh, Chinon etc. can be found dirt cheap and are fun to try out. Just don't expect extremely sharp, technically perfect pictures.
