My current favorite planes are the A340 which I flew with LH to Madras, India a couple of years ago. Their exit rows on that plane have more legroom than I have ever seen in economy class before or since then. I also like the 767-300. I will get my first taste of the 777 during my upcoming trip to China this summer. It will be on China Southern on a flight from Guangzhou to Shanghai. (Hopefully they will become a partner in the DL ff program by then like they're supposed to.) By the way, thanks to everyone who responded to my "Chinese visas" topic thread.
My worst planes are the 727s. The ones that DL uses on its NE shuttle are amongst the nicer ones. Also, the L1011s which are mostly used by DL are terrible. Its notorious 2-5-2 configuration in coach is extremely uncomfortable - especially if you end up right in the middle with two people on each side of you. Also, they're so old - I remember flying them when DL first started its ATL --> LGW route in 1978 or 1979.