ANY character playset - Peppa Pig, Octonauts, Fireman Sam, Ben and Holly etc. The figures are fine on their own but the buildings and vehicles are plasticy shite that doesn't work and falls apart.
Monkey Dunk
Polly pocket sets. The dolls are ok the rest is plastic tat that works nothing like the adverts show.
Play doh sets. The basic dough and cutters are great but the ice cream maker and other sets are impossible for small kids to work by themselves. And they need regular cleaning to get all the dried out bits of play doh out of all the little tiny nooks and crannies 
Aquadoodle. What's the point? Water gets everywhere. Needs constantly refilling with water. DD wanted to just LICK THE MAT and laugh
What's wrong with crayons and paper?
Jigsaws where you get 2 or more 40 piece jigsaws in one box. So you have to spend 30 mins separating the pieces into individual jigsaws before DC can even start. Also jigsaw books, pieces constantly all over the floor because they fall out and you have to spend ages putting them back 
Innotab. Great idea. Battery life is shit. You spend £££ on batteries or spend your life rechanging the bastards. Why not make it with an inbuilt battery that can be recharged with a plug LIKE EVERYTHING ELSE??????????????
Crap character furniture At the end of the day you have to have it in your house. And look at it. Ditto huge hulking plastic monstrosities like play kitchens / shops.
Fijit friend. Just so annoying.