But it is like Watchmen - it's a series outside of continuity. An arc is small scale told within a larger story.
If you're asking what the greatest comic story ever told is, regardless of placement inside a series, Watchmen is high up there. If you're asking what's the best comic arc, Born Again from Daredevil, Year One from Batman, and the Dark Phoenix Saga are common responses.
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ICON:
When there's nothing left to burn, you have to set Bidoof on fire.