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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Yamaha 920 / Yamaha DGX-670 / Maui 5 Go (2) / Senn e935 / Sony C80 / Tascam DP24sd / ATEM Mini Pro switcher (studio)