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None are needed.

Armageddon - Adds a victory condition which adds a great deal of challenge to the game; if you're finding the AI too easy or are just up for a greater challenge, this is a good way to address that. Also makes the campaign into more of a story. Highly recommended.

Master of Artifacts - Adds a couple of spells that allow you to create items for your heroes to equip. Not necessary, but I recommend it anyway because it's just fun.

Power of Serpent - Adds a Great Mage and a couple of perks, allowing you to play with Koatl (lizardman) units. Not necessary, but a slightly different play experience if you want that.

Powerful Lords - Adds a couple of heroes that are perks, so you can start with a hero if you want. I don't have this one, it seems unnecessary to me. Someone else might have a different take.

Return of the Elves - Adds the Arethi Elves faction. This is a powerful faction, especially in the early game, and plays slightly differently from the others (better units but much more expensive.) Makes for slightly more challenging games against the AI, too. Highly recommended.
 
If anyone else is on the fence about these, they're on steep discount on Steam right now; at these prices, might as well just grab 'em all (less than $5 for the set.)