Hi! I'm a viola player too... If you're looking for compositions
in which the viola is the solo player for a concerto there are
plenty:
1) Bartok Viola Concerto for Viola and Orchestra (very
difficult, but exciting!)
2) Viola Concerto No. 1 in D Major by Carl Stamitz (a very
popular piece; basis for most auditions)
3) Viola Concerto No. 1 in G Major by Telemann
4) Viola Concerto in D Major by Hoffmeister
5) Symphonie Concertante for Violin and Viola With Orchestra
K. 364 by Wolfgang Amadeus Mozart (very fun duet concerto!)
If you're looking for Viola and Piano pieces here's a really
good and popular piece:
1) Sonata Arpeggione in A minor for Viola and Piano by Franz
Schubert
And let's not forget the most popular viola solo:
1) Six Cello Suites for Viola by J.S. Bach (Suite No. 1, Suite
No. 2, Suite No. 3, Suite No. 4, Suite No. 5, Suite No. 6)