The only really solo violin music I recall and love are the Paganini Caprices - but these should he heard and seen live for the best appreciation.
Most solo violin music I know are Violin sonatas with piano accompaniment. The best, IMO are actually duets for violin and piano.
My all time favorite is Beethoven's Kreutzer Sonata which I sing along with and have tried the piano part. It gets a bit tough here and their. I also like the Spring, of course.
What solo violin music do you recommend for our personal collections?