A couple things: Okay, Fast Cars was a song that wasn't an additional track on a single, but it's not an album track either. It's not going to appear on an official release in the future because of its placement as a bonus track on HTDAAB. Now the denotation of "b-side" is something of an antiquated definition, especially in the era of the compact disc. The connotation, however, to most people, is going to be a non-album track that is not part of a soundtrack. To me, that means all the stuff released with the U2 Digital Boxed set under the "Rare and Unreleased" heading are theoretically b-sides. That includes Flower Child, Levitate, etc.
Mercy is excluded from all of this because U2 never released it through any legitimate channel, and because it may appear in some form on the next album.
As for my choice of favorite b-side, I'd like comment on Spanish Eyes for a moment. I got the CD single way back when and always loved this song. It's very catchy, it's upbeat, Bono seems to be having fun, etc. Unfortunately, it does sound a bit slight when I hear it. It's not particularly deep lyrically, and it just doesn't have the musical depth of most of what's on The Joshua Tree. Now I know many will say the same things about Trip Through Your Wires, but that fits in a whole lot better with the album's themes, imagery, and sonic setting than Spanish Eyes does. To me the latter would have stuck out like a sore thumb.
You can apply all those things to The Sweetest Thing as well.
My choice is North and South of the River, because it is as musically and lyrically strong as anything found on POP. It also seems to come from not only the same thematic place as what's on the album, but the instrumentation also seems brewed from the same stew. It packs an emotional wallop found on few b-sides.
My second choice would be I'm Not Your Baby, which isn't overwhelmingly powerful, but it's an extremely cool tune, especially the Skysplitter Dub found on the Please soundtrack (that intro with those electronic bird cry sounds just floors me). It's nice to hear Bono duet with one of modern rock's strongest voices. Plus, the drums, man, the drums! Also, I think the lyrics are yet another example of how diverse Bono was in the 90's, and how trite he's become in the new millenium by contrast:
"Cut out the poetry
Let’s hit the main artery
No time for a tourniquet
Let the colors all run out of me"
and
"Daylight’s a kind of robbery
The night is your geography
So you’re not white, you’re pink and rosy
You could be right, but you’re way above me"
Where'd that guy go?
It also sounds spontaneous, like it came easily.
for me
You didn't mess up the thread. 
