Diamond golems have 95% magic resistance. However, black dragons are not affected by this, BUT, - if someone has a vulnerability medal, then it's a different story.

By the way, that's a very relevant observation! Curses work perfectly on diamond golems (one slow is enough to take them down), while dragons have true immunity. The difference is significant. As for the 95% magic resistance of diamond golems, IMHO, it's not very useful. Even AI doesn't use direct damage spells against golems. What can we say about a living opponent?)))
As for my favorite neutral creatures, they are wizards. I remove everyone from the army except archers + one squad for cover, and fill the rest of the stacks with wizards, watch a bunch of blessings being cast, and enjoy it :D :D
Azure dragons are, of course, unmatched in power, but they are expensive and rare. You have to search for them on the map! But with wizards, it's simple - basic diplomacy, and they are already yours ;)