Demons can be much easier to get a no-loss. Besides, they're the best dragon-mincers because of the high fire resistance.
Undead are worse - negative fire resistance, and high posion resistance. However, there are more units dound fire damage than units with poison damage.
That's assuming you're trying different lineups
With lizards, you can always squeeze in a paladin, same is orcs, dwarves and elves.