Whatever key it's written in. I think sharps/flats sound better? I do about 6 songs in C, because it's not a good singing range for me.
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Live: Korg PA4X/Bose L1 Pro 8/Senn 935
Studio: Korg PA4X/Kawai MP7SE/Tascam DP24SD/Sony C80/AGK 214/ATEM Mini Pro switcher/