Lego Basic
Lego Basic is a Lego theme. It later evolved into Creator.
Legoland themes and their successors encourage building the one official model each Lego set is designed for. Room and cost permitting, you can even get multiple sets in the same theme, leaving them all up together to make a larger consistent minifig scale scene.
In contrast, Lego Basic emphasises what's arguably the main selling point of Lego: letting your imagination run wild, building whatever you want to. Any and all scales are encouraged, as is the transient nature of Lego models.
Particularly emphasising this point, the Universal Building Sets reuse the same elements to make multiple different official models, all shown on the box and detailed in the build instructions. This not only provides better value for money, but also requires you to form the habit of tearing down the old to make way for the new.
This idea was so good that when the gear sets evolved into their own open-ended theme, Technic, they continued adding Universal Building Sets of their own. Similarly, when Basic later evolved into Creator, Universal Building Sets evolved into 3 in 1 sets.
Out of all of Lego's themes, I think Basic and its successors most exemplify what Lego can and should be about: imaginative building.
Lego themes: Lego Basic | Lego Creator | Lego Homemaker | Lego Technic | Legoland | Universal Building Set