tough to narrow down, but I'll try
"Kaishou" from Gankutsuou is simply amazing.
"Sora's folktale" from the Escaflowne movie was really good too.
The Witch Hunter robin op, "Shell", is one of those I just can't ever skip over.
"Heaven's Not Enough" from Wolf's Rain, amazing song.
Cowboy Bebop's classic "Real Folk Blues" ending is one of those awesome songs that I can never skip either.
Guin saga opening, "This is my Road" was just beautiful.
Honorable mention goes to Ranbu no Melody from Bleach.