Which artifacts are best to equip a hero with?

Any will do. The main thing is that the hero knows this magic. But in general, mixed sets are the best (Traveler's Book, Apprentice's Book, Master's Book, Archmage's Book, Guild Compendium, Demonology). Or, instead of a book, you can use a staff. That's also a good alternative. For example, a Summoning Staff enhances summoning (it seems to increase it by 15%, but correct me if I'm wrong).