It would be pretty easy to say this is the greatest psychological thriller of all time. The handful of movies that come close always have something a bit cartoonish to them, but what drives Taxi Driver is an unrelenting sense of disturbing realism. It's also aged well; out-there serial killers feel like freak occurrences, but unhinged, isolation-maddened, wannabe-super heroes are all over nowadays.
The Coens got off to a really good start with this one. I think the only person whose thrillers are close to as intense as the Coen brothers' is Scorsese. Anyway, if you like scary movies and you haven't seen this, go watch it now.