It depends on my mood.
Sometimes I like white sugar. A teaspoon will usually do--it's the best place to start, because you can always add more sweetener, but you can't take it away. I don't like my tea as sweet as I once did. Sometimes honey is good. Again, a teaspoon is typically enough, but sometimes I have a sweet tooth and add a little more. Another favorite sweetener (though I usually save it for my hot toddies) is demerara (turbinado) sugar:
I buy it in bulk from the local hippie food co-op.