The dexes with the most shit are probably 1, 3, and 5, but that's likely because their large size drags down the ratings with lots of mediocre-to-shit filler Pokemon like Gorebyss, Jynx, Sawk and Throh, Plusle and Minun, Manectric, Electrode, Swanna, Grumpig, Raticate, Illumise, Druddigon, etc. This also means that they also all have lots of really good Pokemon too, though.
This seems to be an unpopular opinion but the average 'mon quality in the past two gens has honestly been really high, with the notable exception of starters, as well as bizarre swings-and-misses like Crabominable, Bruxish, and Toxtricity.
Overall I'd say gen 6 is the worst: it's tiny dex has very few hits whilst also having loads of forgettable shit like Diggersby, Pyroar, Furfrou, Aromatisse, Binacle, Klefki, Carbink, etc. It's clear the majority of the creature design energy this gen was put into Megas but honestly that really doesn't pay off either: most of those designs are really uninspired add some spikes designs like Garchomp, Aerodactyl, Lucario, Houndoom, or Tyranitar; or just really ugly like Manectric, Sharpedo, Gyarados, Heracross, or Swampert.
