You know when you hear a mum say something "I love all my kids equally" or "I love them all for different reason"? Well that analogous to the way I feel about D&D, apart from it being my offspring. I love the old school killer DM feel of OD&D and the various coloured boxed sets, I also think that 4e is the easiest system to mechanically DM. Each edition has something great about it, they all brought something new to the table - there is no BAD edition.
So favourite? Hmm I guess it would be 4th. But then there are massive problems there too - I guess thats why i'm playing 13th Age. It fixes a LOT of those problems, give me a storytelling game with ease of play and mechanics of newer editions. Im eagerly awaiting the playtest packet for Next, that's shown some real promise, still some issue's but its definitely looks fun.