Not all of the following are entirely bad episodes, some just have ordinary bits in them.
* The Busboy - There's nothing partcularly bad about this episode, it's just that it feels sluggish and uninspired throughout, which did tend to hamper the first couple of seasons of the show. Gradually, the pace of the show quickened up over the years to the right level for several seasons until the last couple of years, where it was way too fast and crammed too much in.
* The Alternate Side - One or two good bits but a pretty lame episode overall. In particular the plotline of Elaine dating a much older man was wasted; perhaps it was something that would've been better utilised in later seasons.
* The Airport - The Jerry/Elaine plotline repetitve and one-dimensionsal, something they usually avoided.
* The Dinner Party - Pretty uninspired, with the low points being the uncomvincing plot element thrown in that George feels physically threatened by Elaine, and the ending to the double parking/Saddam Hussain subplot, which falls completely flat. George's jacket and his complaining about bringing wine make the episode worth watching.
I have to say that the show lost me in the last couple of seasons (especially the final season) but that's more a general complaint against the tone of the show, not specific episodes.