Hero - As has been said, the use of colour is brilliant, and the stylisation - things like the millions of arrows hitting the fortress - are amazing.
The Matrix - The first one is definitely the best, as they don't overstretch things, like with the CGI Neo flying or the Burly Brawl in Reloaded. Bullet time is still incredible to look at, and the fighting is really well shot - you can actually see what is going on rather than it being some ridiculous cluster**** like you get in a lot of martial arts films.
Children Of Men - I loved the long takes in this. Some of them are absolutely incredible, like the one where the car gets ambushed and the building in the end.
Requiem For A Dream - The visual impact of the film really comes from the editing and the incredibly fast cuts. It really adds to the discomfort of the film.
Terminator 2 - This film is nearly 20 years old and it still looks absolutely incredible.
Sin City - At the time, I defended this film to a lot of people who dismissed it as being style over substance. I didn't really understand the complaint, as the whole point of the film is the style. The style and the substance are inextricably linked. The film is a moving graphic novel, almost exactly as it is drawn in the original books. It's brilliant.