I am going to start with what I think are the most forgettable LEGO themes:
The Lone Ranger: The only reason some people still remember this is because is the only Western Theme we got in recent years. Other then that, it only existed to promote a bad movie, and it's arguably forgettable.
Forgettable for the same reason as the Lone Ranger. The only reason it's still remembered is because it introduced the Camel and Ostrich pieces, which never appeared again.
Angry Birds is also quite forgettable, although people still remember it because it's recent. But one of the most forgettable LEGO theme in recent years is...
Egh... this. I don't even know why it existed in the first place. The normal Ninja Turtles sets were pretty memorable, but those based on the 2014 movie were...
Just to promote this disaster movie.
What do you think? What are the most forgettable LEGO themes in your opinion?