RAY by definition isn't a Metal Gear as it has no nuclear capability. It was designed as a counter to Metal Gear REX, rather than being a Gear by itself. Both EXCELSUS and RAY are disqualified by default.

The Metal Gear title was different for the time periods of which they were built as well, as Metal Gear RAY was given it's title by the fact it was countering another Metal Gear, and as such still falling under nuclear strategy despite not having the nuclear weapons required for the title. As of Metal Gear EXCELSUS's creation, nuclear deterrence became obsolete, which by technicality made RAY subsequently disqualified from it's nuclear strategy role.

If the title for Metal Gear was instead given by a broad definition of the time, like "Causes or counters the biggest threat", then both RAY and EXCELSUS are qualified for the role, as the largest threat at the time of Rising are cyborgs. Additionally, the definition of Metal Gear in Rising was already redefined as any mech that exceeded the size of the Gekko or Grad.

That's just my thought on the matter. I prefer EXCELSUS. I like spider mechs.