For me it's Journey to the Centre of the Earth by Jules Verne. It's also a 30-year-old long overdue to be able to read it in its original language. I guess I'll never make it.
I know that this is an unfair pondering question as picking one or more book as a favorite is an impossible task, however for my one book recommendation, I am going to say Son Tiryaki by Müfit Özdeş.