I rarely listen to Tchaikovsky any more, but when I was a child I loved the last two movements of the fourth symphony. When I got to college, I learned to love the violin concerto and the 5th symphony. I never had the opportunity to play his music until after college, when I finally got to play the fourth and fifth symphonies and gained a new perspective and respect for his music.
The main melody of the first movement of the fourth symphony is amazing in its construction--shape, length, rhythm.
Strangely, I've never been able to really get into the 6th, even though that seems to be the one that most people would die for.