By FAR the 4E DMG is the best version for teaching someone to DM effectively, is a fun read and IMO is the strongest of the core 4E books.
The 2E DMG was a complete joke.
3.x versions, while better, were not *that* much better than 2E. I suppose the 3.5 version was better than 3.0 being that it added info from MOTP, ELHB, etc. But it still read like a boring textbook. It certainly didn't encourage me to want to DM the game.
Gary's version I would never want to impy is bad as it's near and dear to my heart-but the fact remains it's a horrible book for teaching someone how to DM. It's a wonderful idea mine though.