For me it depends on the type of talk. But if I'm giving either a lecture or a seminar that involves presenting proofs (or sketches of proofs) then I like blackboards best. There are many reasons for this, but three important ones are (i) it forces me to understand the material well enough to memorize it (I don't use notes for talks, and what I say applies much less to a blackboard talk where the speaker copies from a sheaf of notes) and do the necessary calculations in real time, (ii) it slows the presentation down and gives the audience a chance to digest it, and (iii) what I write lasts for much longer before being erased.
Of these factors, I think the most important is (i): it makes giving the talk a dynamic process that cannot be done on autopilot. I wouldn't say that this cannot be achieved in other ways, and I do give Beamer talks sometimes (but only when they are very non-technical), but ultimately I'm a blackboard person. I have nothing rational against whiteboards, but I like them much less ...
