We're not talking about solo monsters. We're talking about the entire enemy of a rider!

From the ones I've seen, it has to go to the Imagin. They have 0 plan throughout basically the whole season, and only exist to provide an enemy to fight, not to drive the plot along. Even Kai, the final villain, isn't an Imagin (as far as I know).

Second is the Phantoms (excluding Wiseman). Phoenix is cool, and Gremlin has his moments, but of course Medusa is the one in charge, and of course she does nothing.

The Fangires were the worst to me!

The Checkmate Four HAD NO KNIGHT!

The Phantom's were pretty awful, they failed to even be remotely threatening due to the whole thing where they literally never succeeded in making more Phantoms.