Haha funny you should mention that - I've been thinking about this myself. But more along the lines of the themes I'm no longer so happy with. I went through a phase of writing songs based on a sort of obsession with the old religion and its relationship to christianity a while back. I still like the songs, but the lyrics are hard to sing, and make me feel quite self-conscious.
As for themes I like, I think personal experiences are my favourite. Songs about what the writer knows, where you can feel the honesty in them. Rufus Wainwright is a good example of this, songs like Dinner at Eight about his father, and Little Sister, surprisingly about his sister are fantastic. I'm not so good at this approach, but I'm working on it. I also like songs about the human condition (ok, maybe that's every song out there) but by which I mean the likes of Neil Finn's songs - Four Seasons In One Day and In My Command are great examples.
Also, I love the themes of a lot of Tom Waits' songs - which tend to concentrate on the darkness we all hide, and vagrancy, marginalisation and so on. Great stuff!
I can't really say I like to write about a single theme more than another. I tend to write whatever comes out - those songs tend to be quite heavily disguised very personal experiences. Loads of metaphor and ambiguity! I think my favourite lyric/ theme has to be my song Pilgrim, which is about the journey into our own darkness, and how acceptance of it can actually be quite liberating, and even beautiful, if somewhat disturbing!
Great thread, this!