https://pokemongo.gamepress.gg/best-attackers-type has some information. In summary:
1. Mewtwo and Groudon are very safe.
2. Many Dragon-type legendaries in Gen IV, but you would want to power up your current Dragon-types to raid them.
3. Ice-types are under great threat from Mamoswine.
Wonder why that article does not list Darkrai as a potential threat to Tyranitar.
If gym defence matters in your area, rest assured that Blissey is a safe investment.