The original, by a wide margin. Part II was fun, but a little too inconsistent and disjointed in its quality. Having the contrasting directorial styles integrated into one film just couldn't match the polish of the first.
And the first is just a classic. Krypton exploding, Clark outrunning the train, the majestic Normal Rockwell-esque wheat fields surrounding the Kent farm, creating the Fortress of Solitude, "You've got me? Who's got you!?", the nukes, the earthquake, and of course, reversing the earth's rotation to turn back time.
Gene Hackman was a much more charismatic villain than Terence Stamp, and Christopher Reeve's Superman underwent a significantly more fascinating character journey than in the sequel.
There are a number of great moments in Superman II, but the first was definitely better.