Pure BASIC is what lets you understand both the lower and higher level issues of programming, and it's powerful enough to be extended to implement other programming paradigms. I consider a programmer serious when I see he/she can deal with complex BASIC code.
Of course, then you should try languages from other paradigms. I'd suggest to try at least a bit of all OOP, event-driven programming, functional programming, and more bizarre stuff like production systems, just so you fill your mind with ideas.