ice types

Cloyster's and Walrein's attack isn't that bad. Cloyster's is 4th highest of any ice type though Sneasel is useless so 3rd realistically, and it's only 6 points behind Articuno.

It's only Lapras that lags significantly behind the others in DPS. Overall I'd say Walrein is the most well-rounded ice type behind Articuno. Decent attack, good bulk. Not any glaring weaknesses. Only has a 1 bar charge move but Blizzard is still a good move.

For raiding I'd just send in the ones that do the most DPS and have a second team prepared or max revive the first team.

I know you’re looking for Rayquaza counters, stack up on some Kyogre with Blizzard. Ice types are mostly irrelevant do to there being no incredible ones. Kyogre with Blizzard (or any strong Pokémon that’s super effective against Rayquaza will be fine.

If you live is a rainy climate, grind Spheals for a week and evolve your best ones. I’ve found several over 750 CP in the rain.

Cloyster with avalanche.

Outside of Articuno, they're all very comparable, basically just a sliding line where you either trade tankiness for DPS (Jynx) or at the other end of the spectrum, trade DPS for tankiness (Lapras).

Looking at sims against Rayquaza, I'd favor Piloswine out of the non-legendary ice types, unless going for a Jynx glass team. Very good DPS, respectable TDO.