I'm not sure there is a real answer to this question. The worst plugins will be:
A-outdated
B-[*******]
C-installed incorrectly(maybe to the wrong drive or folder)
D-The last type that can give headaches are those that are really power hungry(maybe coupled with a computer that is under-spec'd). An example of this is KeyScape(specifically their Yamaha C7 grand). My system(details below) runs great under most conditions, but that plugin will drag it to its knees. My final solution was/is to use MiniGrand while recording and editing midi. Once that is all good, I swap plugins to KeyScape and commit the track.
Ozone is another power hog.