Humanoids, Outsiders, and Undead.
Humanoids can have really compelling plot hooks on them.. like correspondence found in a pocket that details the scheduled time, two weeks from now, that the forces of Evil are planning to attack (PC name)'s town and enslave anyone they don't kill. And where the planned staging area is, just before the attack.
Outsiders for their black and white malevolence. Demons are quite clearly Bad. Everyone in the party can easily understand why a demon infestation needs to be prevented/nipped in the bud. (Some parties need a lot of convincing to stay together.)
Undead quite clearly plan ahead.. A Lich can Charm, Dominate, and pay for quite a few agents, and do a lot of its own information gathering. Some of them even pay for "special information" to be hand-delivered to far-away places by adventurers that seem a bit too nosy.. and have already planned for the recipient to be kidnapped and taken to a place where creatures inimical to the Lich's goals are thriving. After all, everyone knows what Adventurers do when presented with obvious foes, right?
The best part is when the unconscious recipient is unshackled from the altar, healed to wakefulness, and given the scroll that's travelled so far to reach them.. and they get a look of puzzlement, and exclaim, "Betrothal? What? I don't even *have* an Uncle Mandor.."
Liches even have time to research the special form of command-detonated Explosive Runes which only goes off when the phrase, "We've been duped!" is uttered nearby. Just priceless.