In the Gamepress Mewtwo raid counter. Dragonite is said to be good but there is no mention of Rayquaza. Is there a reason for that? Surely anything Dragonite can do Ray can do better?

Old guide

rayquaza is to glassy vs ice beam.

Several new pokemons are better options such as SA Moltres.

The guides is more of a guidline taking into consideration what people have. It recommends double dark gyara despite it beeing a rather bad option.

Truthfully you're better off looking at the solo/duo/trio guides over the standard counters lists. Mewtwo in particular has a handy chart to size up counters based on what moves it knows, but it needs to be updated for the SB/HB loss and IB/FT/TB gains.

I wouldn't use Dragonite or Rayquaza against Ice Beam Mewtwo. They are fair against Flamethrower.

Best counters are SB Mewtwo, B/C Ttar, SB Gengar (good against PC/FB, so so otherwise). In windy weather, Ray can nudge up into that list, rainy lets Kyogre play. After that, everything is third rate.