like everyone else, it's impossible to pick one - I'll go for a short list of 10 at this momenti, probably clouded by recent listening...
1. How Does The Grass Grow
2. Sunday
3. Valentines Day
4. Thru' These Architects eyes
5. Goodbye Mr. Ed
6. The Next Day
7. The Glass Spider
8. Afraid
9. You will set the world on fire
10. We Are The Dead