Agree with the Brahms and Walton suggestions. Note also Brahms 2 songs for alto and viola and Walton's suite for viola and small orchestra.
Then there are the Mozart duos for violin and viola, sinfonia concertante (for violin and viola) and Kegelstatt trio.
Bach's Brandenburg no. 6 (for 2 violas).
Lots of wonderful string quartets - but Dvorak and Britten make particularly good use of the viola.
Speaking of Britten (who was no mean violist) - Lachrymae is one of my all time favourites and not just in terms of viola pieces. I have a lovely recording by Kim Kaskashian on an album called Elegies which also includes superb works by Vieuxtemps and Wieniawski. Then there's Britten's Elegy, written when he was 14 I think.
Shostakovich Viola Sonata (one of his last ever pieces) is stunning. Get the recording by Nobuko Imai and anything else by her.
The Hindemith sonatas are an acquired taste but I like them a great deal.
If you really want to push the boats out, there's the Berio Sequenza.
Oh and if you pay attention to Mahler and Sibelius symphonies there are some great passages for viola which demonstrates what it is about it that blows those violins and cellos off the platform.