God I could write a thesis or some shit on this, there's so many things that make someone peak zonkbait!
For me personality is one of the biggest things about what makes good zonkbait. here's a couple examples:

The more powerful, serious and distinguished someone normally is, the more delicious it is to see them get zonked out of their mind, grinning stupidly. That's your Von Lycaons, your Bagheeras, your Raurus

I'm also a fan of goofy stupid boyfailures who just don't have their shit remotely together, and are prime material for getting 'fixed' through brainwashing. Think like Weekly, Rigby, or to give a recent example from Wildwood, Dmitri