Classic-Castle.com

I only like the Classic Solid Stud Smiley Faces. Not too detailed nor too ugly. All my minifigs use these heads except for the bad guys in my collection.

SwiftBlade

My favorite is the classic smiley head. I have to use it for both male and female minifigs because the only female head I own is this: http://www.peeron.com/inv/parts/3626bpx250.
And I don't like it that much, so I never use it.

How about the head used in the studio-series, you know the one that's printed on both sides, one happy, one sad/afraid, I never liked a prisoner that sniled like that when he was drawn away to the dungeons:

Wow this is a really old chat, lots have changed since 2004. Now, there are countless ones to choose from with a wide range of emotions.