Around 1990.
LEGO still had the status of the best and favourite toy, but the new sets were not that great at the time.
From the year 2000 I liked football, Adventurers and RC car. I remember I was dissapointed about most themes. KK was probably the worst line in castle until that time and Town was really terrible compared to before. Pirates were not even there if I am not mistaken.
I have noticed It is almost only city and licences now. When HP and SW came out I was appaled. That was not LEGO for me, no creativity, only brick models of some other toys/films and total collapse of the LEGO universe. Imagine if the next Star Wars would include Emmet Joseph Brickowski as the main character or a Harry Potter movie about Harry getting a monorail for his birthsday.
The builds and techniques have improved a lot, but the most sets have no creativity, there is imagination and fantasy lacking in the (most) themes. There are no alternative builds at the back (of course not, if it is a licenced model) and everything is so boringly structured, all characters are different and have names and stories etc. Yes, there are some great sets in Ideas and Icons, but you really have to be into LEGO to even find them, usual sets on the shelfs are so boring and repetitive. If someone does not like city, than there is not much to choose from. Especially in countries where American culture is not that strong, LEGO became a toy with very little appeal for the most part. There are no historic themes, although the possibilities are limitless. They are focusing on quantity, but it is mostly the quantitiy of licences and remakes.
Designwise yes, as the builds in the early 2000s were mostly clunky and weird, but everything else is worse now. Even the best design does not help, if there are no new ideas to design from.