I did O&O Wolgan Valley in 2015. Very good overall, but a clear step down from Saffire in service, F&B, and a few other areas. Spa was spectacular and I loved the villas, but not at the level of refinement of Saffire.

Haven't been to Reethi Ra, but given the high room count I would be highly unlikely to choose it over Cheval Blanc or Velaa.

I did Six Senses Zighy Bay back around 2010. Absolutely loved the setting, beach, and water, and the Senses on the Edge restaurant was great (at the time). But F&B at the other venues was well below average and the main pool areas was just....sad. And far too many villas for the space, so unless you have one directly on the beach not a great set up.

Frankly I don't think O&O or SS really compare with Aman (even the "declining" Aman that so many are concerned with)....