The Descent Into the Depths / Against the Giants / Demon Queen series is the best arc, IMO, the first ever 'epic campaign.'
Keep on the Borderlands has a special place in my heart, as does Barrier Peaks.
But the overall best I think I'd have to give to the Slave Lords series, and that's because the Slave Lords themselves were so darn memorable. I think strong and memorable characters, like Strahd or Lord Soth, can make an otherwise mediocre adventure shine.