i would speculate that brahms would have liked beethoven's 9th violin sonata the best. the 'kreutzer.' that is because brahms liked to make his phrasing sound like that of the violin or voice. you can listen to this masterpiece if you google 'beethoven's 9th violin sonata' and then listen under wikipedia's recording.
somehow, i always think of brahms as 'holding in his feelings' while at the same time writing the most emotional music a composer ever has. his german requiem is so amazingly taunt and yet spills over this lovely melodiousness that beethoven, mendelssohn, schubert, and schumann had. with him - it just goes on forever.
Hugo Leichtentritt (whoever he is) gave a lecture at harvard and said this: 'bach is his great-grandfather, mozart and beethoven are his two grandfathers. schubert is his uncle. mendelssohn is his elder cousin, and robert schumann is his father.'
if i remember right - brahms was somewhat self-taught as a pianist and used to 'hang out' at the schumann household both as a youth and in his later life. i'm sure he picked up some tips from clara - and robert probably discussed composition ideas with him. schumann probably remembered how hard he had it - to make a breakthrough with clara's father mr wieck - and robert schumann was not hardened towards young brahms when he made his first entreaty for whatever it was. whether lessons or friendship or whatever. i can't remember how they first met. despite the growing awareness that schumann had over brahms adoration of clara - i think he truly accepted brahms and never troubled his thoughts over him. it seems that these musicians found ways to relate to each other through their music. floristan and eusebius took on a hidden meaning that was both two sides of robert's personality - and also distinctly robert and johannes.