Regice outdos every Ice type by virtue of its bulk. Even with the weaknesses, it has titanic bulk that I believe IS higher in the game counting those weaknesses. This will not only make it a fearsome raid boss but also a strong Pokemon. Walrein might be better than Lapras thanks to Avalanche access, but that remains to be certain.
Can we remove CP as a metric please lol. We all know the raid bosses will be something they'll do with a theme in mind (starters, trade evolutions, etc.):
Tier 1: Gen 2 or Gen 3 middle starters, maybe Metang / Shelgon / Vibrava that would massively help rurals
Tier 2: Camerupt, Sharpedo, Breloom | Zangoose, Seviper
Tier 3: Shiftry, Hariyama, Claydol, Cacturne, Grumpig, Altaria
Tier 4: Blaziken, Swampert, Sceptile, Milotic, Walrein, Aggron, Flygon, Gardevoir
Stuff that could go either way: Ludicolo, Exploud