Tough call...
Favorites:
Academy's 1/72 ME-163 Komet
(It practically falls together, has a nice towing tractor included, has respectable detail for the scale and needed no filler putty anywhere.)
Revell Germany 1/72 LeClerc 5 MBT
(This one falls together too, nice clear instructions, a good introductory kit for armor newbies like me)
Losers:
Any pre 1990 Hobbycraft Canada product
Revell 1/72 Blue Angels diamond formation F-4J set.
(Four crappy Phantoms on posts so flexible they couldn't hold formation to save their lives)
Airfix 1/72 solid nose DeHavilland Mosquito.
WAY TOO MANY PROBLEMS TO MENTION!!!