While I have always been fond of the undead as an enemy (I ran an 11-year campaign with the undead as the main enemy), the Lich is the one with the most versatility. Most undead are fairly mindless and can, at times, be uninteresting. The lich is an undead that still retains it's intelligence and cunning. They can serve as a single encounter or be the masterminds behind an entire campaign. Their challenge level also ranges from the baseline to almost god-like with the added abilities of magic and items.