Mine has always been the Bolero of Fire. It's just a brilliant sounding tune. The final ending notes on it are simply beautiful. In fact, I'm listening to it RIGHT NOW! (Cause of this thread of course )
Those tunes are very catchy, being bolero was upbeat and I could feel myself getting pumped and ready to charge, the serenade was rather calming and very meditative, so it made me think clearly. The Song of time however, was the most soothing. I was able to stop for a moment and relax.
My favorites are the Requiem of Spirit and the Serenade of Water. I like how both songs sound and how their tune fits well with the element that they are representing. The other ones are awesome too, but those two are the ones that stand out the most to me.