FHR (AMEX) is usually recognised the most by hospitality providers. The biggest chance to get an upgrade ( upon availability), breakfast for two if booking a double occupancy room, property credit(usually $100 or £85, noon check in (upon availability), GUARANTEED 4pm check out which I consider their main benefit, some properties offer lunch for two, dinner for two, massage, etc.This depends on each property.

Virtuoso - less likely to get upgrade as there are way too many Virtuoso bookings, noon check in/late check out upon availability, property credit - usually £60, breakfast for two, the end.

Signature - good luck with getting upgrades( upon availability ), the same as Virtuoso for check in/out, credit usually around £40, breakfast, usually continental only.

All this varies by each property, hope this helps.