It's pretty easy for me: Gershwin's "Summertime" as I said in the I need it" thread. The best version I've heard of the song is the one by Ray Conniff and his orchestra.
It's of course hard to say what's the best song you've ever heard, 'cause "Summetime" wasn't my favorite song 6 years ago. Then it was probably "Black no. 1" by Type O Negative. 2-3 years later it was maybe "Step into a world" by Krs-One and then a year later it was probably "Flashlight" by Parliament. It's also got to do with that I've had periods where I listened to certain kinds of music. When I was in 8th-9th grade I only listened to metal, when I was in 10th grade and my first year in college I listened to hip hop, then I started to listen to 70's funk and now I listen to more or less every kind of music that existed before the 80's, so my favorite song has changed quite a bit. But I think it's fair for me to name "Summertime" as my all-time favorite, as no song has "touched" me in such a way as this does! I still think the Type O song is superb (and they're still my favorite band), I've grown tired of the hip hop shit, the Parliament song is still superb (although I rarely listen to it), but I have a hunch that "Summertime" will still be my favorite song in 20 years. Although the "Flashlight" song is goddamn groovy and shit it's not something that I wanna listen to all the time. When it's Friday nigth and I wanna go drinking maybe I'll put it on. "Summertime" is "universal". I could listen to it Monday morning if I'd like and Wednesday afternoon. Okay, I usually won't listen to it Friday night (unless it's Billy Stewart's funky version), but besides that it's got the quality to stay fantastic over the years that the hip hop shit didn't have and the funk stuff only partial had.
The list I posted in the "I need it" thread had Bill Withers' "Ain't no sunshine" as no. 2. I also have a couple of different versions of this song and they are also great, but not as great as Bill Withers' original and not in the same way as "Summertime". I more or less have a guarantee that "Summertime" will be great when I buy a record with it. Even Klaus Wunderlich's version is great! Once again it's "universal". Everybody (except the Ohio Players) can play it and still make it good. A song like Procul Harum's "A white shade of pale" is also fantastic and in my top 10, but I've heard a couple of cover versions of it, but they were all gross and horrible!
"Summertime" = No. 1 FOREVER!!!!!!!