It was Gen 4 for me:
1.Starters were kind of bad
2.All Pokémon were too... "tacky"...
-I mean... Take the final starter evolutions: Tortera - had too much stuff: a tree, spikes and stuff just TOO MUCH DETAILS; Empoleon - the trident on its head and the finns and the white thing on it's belly!; The monkey that I don't jnow the name of - the burning head, and those armour-like things... Did you get what I mean - it was TOO MUCH!
3. There were a lot of bad Pokémon (my opinion)
-Examples: The cat that has UGLY in it's name, the oven/landmower/fan/washing machine/fridge (I like rotom but not its evolutions), the ballon, the duck with hoola-hoops (Sorry but i just don't like Cresselia)
And: The anime episodes were full of fillers - more than usual
...
Although, there were some good Pokémon that came in Gen 4 - Shinx, Riolu and a couple more + some evolutions