Lego System is clearly the best theme, I haven't seen Lego top that recently.
Lego was like, hey you can combine castle, pirates, space, wild west and town sets together with Lego System. It made sense because Time Cruisers/Twisters were in Lego System and time cruisers/twisters could go back in time. So technically they are all one theme.
Lego hasn't done anything like this recently because they rely too much on licensed sets now like starwars, marvel, etc so it is not one big theme anymore.
https://www.youtube.com/watch?v=1BVrnM7ZZcg