Billy+Mandy?

I don't recall them ever having a thing during the course of the whole show's run. However, there were times when they did seem to really care for each other, mostly in life and death situations. I remember in the episode where The Grim Reaper shows them a possible future where, in a thousand some years, Mandy has become a giant slug kind of creature and now rules the Earth that she has cloned Billy millions of times in need of company so while it may seem that Mandy utterly despises Billy and is frequently annoyed by him she does look at him as her only "friend". However, I don't think Billy and Mandy would ever go any further than being "friends" in their relationship. Girls usually dig guys with self control, patience, and who like to take charge of things, neither of which Billy possesses. He is a complete buffoon who has no self control and couldn't even take command of anything in any serious fashioned even if he tried. He's also so slow it could take him quite a large amount of time to really understand any particular subject. He's make a terrible boyfriend and even worse husband especially given his low intellect and hyperactive nature.

...Doh!