You know what I love? Villains. Being a total action-adventure lover, villains are a must and in my mind, there are four types:
The Crazzies: Male or female, these people-- humans or otherwise-- are driven by one thing and one thing only. Be it revenge or world domination is up to you. That thought consumes regard for human life, empathy, and even self preservation (why do you think they keep accidentally kill themselves at the end of the book?).
The Calm, Cool, and Collected: The "leader", usually of a group of baddies (but not always), who never freaks out no matter how bad (or good) the situation gets. They almost never mess up and know exactly what they're doing. They're in control, suave, confident-- until their plans are foiled, anyways.
The Ones Behind the Curtains: The ones usually controlling the "front" bad guy (usually one of the Calm, Cool, and Collected variety). They're smart, patient, and sometimes a little off their rockers-- in other words, they might just be the scariest, so watch out, heroic characters!
The Soft-Hearted: More likely than not, there's at least one baddie (usually just one) who is more or less unwilling to do their job 'cause they actually have a soul in there (and are more or less willing to help a protagonist out when they've been captured-- yay for soft-hearted villains!).
Of course, these are only generic catagories. Got any more? My ears-- and comment lines-- are open! Send in a suggestion!