Digging this out from the dead cos I want to answer it myself. 🙂
I'm torn between two, so I'll do it thusly. My favourite British series is This Life. This Life was a drama ostensibly about a bunch of twentysomething lawyers, but mostly revolved around their lives outside of law. It's seen as something of a seminal television series, and quintessentially mid 1990s. It's funny, smart, and involving, and also quite nostalgic now.
Best American series has to be Twin Peaks. Utterly nonsensical in parts, but also desperately cool.