Bad is played in the key of A and many fans believe it to be Bono’s best vocal performance. Personally, I prefer the Dublin ZOO TV performance because it has a more mature sound. He hits a C# as he sings “I’m WIDE awake” in Bad. This is not falsetto folks. This is pure and unadulterated head voice. And let us also not forget that he was hitting that same note during the ZOO TV tour, except I think they were tuned down half a step. So, it’s really not the same note. But impressive none the less.
ISHFWILF is played in the key of Db. The chord voicing is D but the guitars are tuned down 1/2 a step. It was played in B in some of the live versions but not the studio version. The highest note he hits is an A# and he hits several of them during the verses and not during the choruses. The ‘still’ is a G#.
In Red Hill Mining Town he hits a C. “I’m hanging on, your ALL that’s left to hold onto…” I’m pretty sure I read that they didn’t perform this one much because it was so high for him.
Bono hits a C in Wild Horses, "Come on now love, don't you LOOK back."
Bono actually hits a B in Kite. “A man WHO sees…” But they play it a half step down in concert.
Stay is an A# as was pointed out. “STAY, and the night will be enough…”
It looks like Bono’s head voice threshold is between A#-B-C-C#. As far as the best note, we’ll have to let the populace decide. My personal favorite is Stay but the staying power of SYCMIOWO is very impressive. But, since it’s an A I’m not including it in my list.