There aren't many viola concertos. The Telemann that Jaina mentioned is by far the most well known, it's fun to play, but more fast and rhythmic, I wouldn't describe it as beautiful or moving. Schnittke has a modern viola concerto that's quite cool, but it IS modern... not sure if that's your taste or not. I've seen the famous Elgar cello concerto transcribed for viola, that's definitely beautiful.