Let me add to this that I am not singing it at all and I play it in F, because that's how I found it in The Real Book. Besides that I find it a pleasant key to play in - at least for this song.
How it was originally written is interesting to know, but Jobim changes keys too, probably to accommodate Astrud Gilberto or/and Stan Getz. I think if I would have to sing it, I also would find the best key for me to sing in. (and I am sure I would try to sing it in two languages, Brazilian and English.) Unfortunately I don't sing.
The chords I use are pretty much the same every one uses, with some changes. The only big difference is that during improv I play the bridge section:
F#M7/F#m6/F#m7/F#M7b5 So the notes go:
f#-
a#-c#-
ff#-
a-c#-
d#f#-a-c#-
ef#-a-
c-e
then the same as during the melody:
BbM7/Gm7b5/Am7/Db79 (not b9)/Gm7/C7b9//
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drdalet