Ace of Spades has to be one of the most air-guitared riffs in history.
The riff isn’t too hard to play on the uke. The only problem is the speed. Because it’s fast, it’s much easier to play it by strumming rather than fingerpicking (even when you’re only playing one note). Make sure you have the other strings muted (the G-string with your thumb and the E and A strings with your fingers).
The strumming pattern is down, down, up, up, down for the single notes (and all downs for the double notes).