Best to worst (IMO)

OS:

Yamaha (by a wide margin)
Korg
Roland
Ketron

Sounds:

Yamaha (overall)
Korg
Roland
Ketron

Styles:

Yamaha (except drums & bass)
Korg
Roland
Ketron

Vocalizers:

Korg
Roland
Yamaha
Ketron (don't know)
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Yamaha 920 / Yamaha DGX-670 / Maui 5 Go (2) / Senn e935 / Sony C80 / Tascam DP24sd / ATEM Mini Pro switcher (studio)