When I walked out of Watchmen I was thinking 'this is better than TDK', but I'm watching TDK right now on dvd and am not sure, having only seen WM once I find it hard to compare which I like more right now, boring answer but true. The true test will come with the director's cut of WM, which is the true finished film.
Jackie Earle Haley is as great to watch onscreen as Heath Ledger though, I'd say that was a definite, wish there was an option to watch the movie with his face showing all the way through, would love more Kovacs in prison on the DC, the way he played the death scene was beyond amazing, one of the best scenes ever.