I suggest lava. I think that would look best.
I'm planning to build the lighthouse in Spawntown of glowstone from the first floor up, but then again, there should be something distinguishable at the top, probably, and a glass thing filled with lava sounds like a good idea for that.
If you want uniqueness from Spawntown's lighthouse, netherrack would be a good choice.
Or maybe just wafer. That sounds like an... Interesting... Material. I'd like to learn more about it.