I've been playing for four or five years; I'm a freshman in high school right now. My favorite pieces are Tableaux de Provence (although I still can't play the pesky fifth movement up to tempo), Bozza's Aria, Gurewich's Concerto (it's simply a fun piece to play), Creston Sonata, Jacobi Sonata, Glazunov Concerto, and Introduction and Samba.
Introduction and Samba is probably the hardest out of any of those. Insane altissimo, tonguing, and runs. It's very hard to make it all sound good, even the melodic parts.
YouTube - Irmo High School Symphonic Winds