Most desirable would probably be a good sense of humour. I've built most of my life around being able to make people laugh, and I can't stand it when people are utterly incapable of making me laugh. It's really not hard to make me laugh.

Least desirable ... definitely someone with a shady character. You know, those people who brag about doing nasty things behind people's backs, or you witness for yourself how they go behind people's backs ... Lying, sneaking around ... Ugh, just completely unbearable to me.