Monster > Undead
The Undead are, marginally, the most common enemies in the game. They appear in all Acts - in one form or another. There are three different types of Undead, as well as a wide array of Undead heroes.
As a general rule, they're either very resistant to or completely immune against life leech. Skeletons are highly resistant to elemental and piercing damage and immune to Life Leech.