TNG didn't have too many bad episodes. A lot that people don't like I actually enjoy--Rascals, Genesis, Imaginary Friend, Disaster, The Game, Shades of Gray, Qpid, Menage a Troi, The Loss. And "Sub Rosa" has grown on me over the years--it isn't good but the Irish gothic romance novel atmosphere is appealing.
Ones I didn't care for were Emergence, Masks, Cost of Living(but even it had its moments), Eye of the Beholder(interesting episode until the big reveal it happened in Troi's head), Birthright just a boring and pointless two parter, Rightful Heir, Times Arrow II was a disappointing wrap up that wrapped up too quickly and barely mined the story potential, Firstborn, Force of Nature, Bloodlines.