Lowest fly-by ever?

That's certainly low. Somewhere in the webosphere there's some footage of a trio of naughty French mirage at somewhere like Solenzara, and one of them is bloody low too.

Anyone got the photo of the Typhoon at Farnborough a few years ago that scorched the grass at the bottom of a loop? The giveaway is the canards at maximum alpha and the brown streak the pilot leaves when climbing out...

I 'raise' you 1 x Buccanner. Apparently the Bucc could do this on a regular basis as it formed an air cushion under the aircraft that prevented it from stoving in at these heights.
They get a lot lower over here, the A10 at Rammstein apparently flew a circuit in someones cellar, the Airbus team then had to top that by demonstrating the clipping of tree roots from the underside at the Habsheim airshow!
Just a couple of many video's on YouTube.