What is the best wheel chair cushion?

Rojo. Typically the cushion should be patient specific' and a rojo cushion is a pretty good one. Hopefully, the individual underwent pressure mapping to assure a proper fit; an improper fit might lead to skin breakdown and pressure 'ulcers' which require medical intervention.
No "best" The best cushion is one that is comfortable, provides even support to avoid pressure sores, does not "bottom out" is durable, washable. It is best to try different brands to find out what works for you. A physical therapist is the best person to help you with this.