Terry Pratchett! Read everything!!
Ahem. I think I disagree with Ann–The Color of Magic is the very first Discworld book, but it’s definitely not the strongest. I like his Night Watch series best, which starts with “Guards! Guards!” though you could certainly get into the next one, “Men at Arms,” without reading it and it’s better. “Equal Rites” and “Mort” are also good places to start.
However, those aren’t YA, just straight (excellent) fantasy. I believe the Tiffany Aching books (“The Wee Free Men” was the first, I think) were his first YA, and I’m told they’re excellent, though I haven’t read them myself.