Arial Unicode in general purely out of its functional capacity — displays many glyphs that would otherwise appear as blank blocks, and they're shown clearly.
Arial Narrow for assignments, letters and the like. It allows me to fit more on a single page, and I like to keep such things to one page if possible.
Eurostile on Windows Messenger... I'd imagine I just find it most comfortable to be typing in, and again it's rather condensed.
My actual favourite, on the other hand, is fixedsys, but it's highly restricted in what glyphs it can display and is not especially easy on the eyes for reading. Ultimately when it comes to fonts I'm function over form, hence the list above.