Open spoilers for Buffy, Angel, and Preacher. Buffy spoilers lurk in this OP. You’ve been warned.
They’re badass rogues with British Isles accents who have complicated love/hate relationships with the nominal stars of their series and a fondness for grand redemptive gestures. And they’re vampires. And they like to fight.
Me, I’d vote for Spike, except that he’s vulnerable to a stake through the heart whereas that’d just slow Cassidy down. And while Spike hasn’t been legitimately staked yet, Riley frickin’ Finn managed to get a “pretend” stake through his heart at least once. So his ability to soak up more damage gives Cassidy a slight edge in my book.
Then again, maybe they’d end up as drinking buddies.