As a product of the 70's and 80's I started out watching the Roger Moore ones and really loved them. Then I got to the Connery ones and grew to like them almost as well. Slowly as I got older, Connery replaced Moore as the ultimate Bond - and From Russia with Love became one of my all-time favourite movies. Now, I think the Moore ones are just silly. And he was waaaay to old in the last ones. Handsome as hell back when he played Ivanhoe and Simon Templar though.
I remember seing George Lazenby in OHMSS as a kid and just found it boring and wierd. However, when I reviewed it as an adult I realized it was a really good movie and Lazenby was a really good Bond. It would have been fun seeing him in more movies, but I fear he would have gone down the Roger Moore route anyway - the sillynesss was just a sign of their time.
Timothy Dalton was obviously a typical 80's action-movie Bond with not too over-the-top plots. I think his two films were okay, but nothing more. He was considered back before Moore, but I think he didn't want to do it. I think Dalton would have better in the 70's, than the late 80's
Brosnan was originally supposed to have taken the mantle before Dalton, but couldn't on account of his contract to do the Remington Steele TV-series. When he finally did get the part, he was really good, but suffered something of the same thing that Moore did - campy plots. Brosnan is something of a hybrid of all previous Bonds. He had the sex-appeal and badassness of Connery and the British elegance of Moore.
Daniel Craig was great in Casino Royale, and the movie had a down-to-earth, back-to-basics feel. I have pretty good hopes for Quantum too. He is more a Connery type and I don't imagine he would do well with Moore-sillyness at all. Based on one movie alone, I'd still rank him pretty high.
So, my final list of best Bonds are from worst to best:
6. Roger Moore
5. Timothy Dalton
4. Pierce Brosnan
3. George Lazenby
2. Daniel Craig
1. Sean Connery
Now, if they only could make movies in the 1950's/1960's era, during the cold war, where Bond really belongs.