I'd hate to give up all the other composers, but it isn't hard to decide. Actually in spite of my user name, it's not Mahler I'd pick (whoa is me I'd miss him!!!)
I would pick Brahms. He has high quality music in pretty much every subgenre. German Requiem is amazing, then you have his wonderful piano concertos, violin, cello & clarinet sonatas. The wealth of other chamber music, piano works. Yes, I'd be ok if I had nobody but Brahms.