Well, naturally there'll be winners and losers in every season if you bother to give them rankings, tiers etc. of some sort.

Technically speaking, Hyouka was my biggest waste of time because I already knew so much of it beforehand. However, within the seasonal shows I watched (with the exception of Saintia Sho, if you count it), Gridman is my worst, but only because it took so long to win me over. The first time it truly won me over was episode 9 - I only continued past episode 2 because of Samurai Calibur (best boi in this series, even though I used to think Vit would surpass him).

Gridman's case is more of a waste of time than something that caught me straight away and only botched the landing (episode 10+ for Double Decker), because I felt more ambivalent towards the former for a longer period of time than the latter.