[Dys]

Jayce and the Wheeled Warriors. The vehicle toys weren't that bad for the time, problem is they didn't really put any effort into tying in with the supporting fiction, the human figures were generic space heroes rather than the characters from the show and the Monster Minds were nothing but vaguely faecal looking green lumps.

I know the show came after the toys, but Mattel could have tried harder, relaunched the line after the show had gone to air with better pilots, or figure packs of the show characters but they didn't, so that was that.

I'd also like to vote for Transformer: Energon. With combiners, it's tempting to be a completionist to try and get all possible combinations, sadly the method they chose for the Energon combiners left a lot to be desired. The individual Autobot figures were basically either crippled, or had ridiculous proportions for a combination scheme that looked terrible and included no new parts, just vaguely moulded heel-hands. They were basically a step back from the Landcross bots, done over a decade later, leaving out the ability to make a super robot if you collect them all.
They should have gone with the basic-deluxe combination scheme from the off, which they showed they could pull off at the right pricepoint without crippling the Transformations of either bot.