When I think "story arc" I think "segment of events within a larger coherent narrative." So for examples, "Blackest Night" in Green Lantern, "R.I.P" in Morrison's Batman, or "Cell Saga" in DBZ.
TDKR and Watchmen are a lot more similar than you think, since they are both standalone stories that don't require much or any knowledge of the characters or setting outside of that one book. So they don't really fit that definition IMO.
Also, no, there are plenty of comics better than it, although it is a great read.
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