My vote's with an international magical train flying across the English Channel.
We know that Hagrid flew across part of the sea in Philosopher's Stone, to collect Harry from the hut on the rock, and as far as mass goes, Hagrid is a heavy load. Yet, he is able to fly:
‘How did you get here?’ Harry asked, looking around for another boat.
‘Flew,’ said Hagrid.
‘Yeah – but we’ll go back in this. Not s’posed ter use magic now I’ve got yeh.’
They settled down in the boat, Harry still staring at Hagrid, trying to imagine him flying.
Philosopher's Stone - Chapter Five, Diagon Alley - Pages 50-51 - Bloomsbury
Now Quidditch Through the Ages says no spell has been invented that allows for unaided human flight (QTTA, page one, chapter one).
However, a train is not a person, and we see many examples of objects flying in Potterverse: brooms, Golden Snitches, Christmas ornaments, certain magical beasts, etc.
Furthermore, both Voldemort and Snape can fly in human form, using an unknown form of magic, which contradicts Quidditch Through the Ages.
We know that items enchanted to fly can also carry humans (the broom is the obvious example), so it's entirely possible a train -- a vehicle -- could be enchanted to fly across the English Channel while carrying humans. This fits most nicely with the rules of flying in Potterverse: while there are exceptions, the law of flying seems to favor magical objects ferrying human witches and wizards.
I do like the idea of a giant Floo fireplace in Platform 7 1/2, and seeing that train go from 0 to 200 with a bang and hurdling through a cascading fall of Floo powder, and reaching Europe in the blink of an eye! I'm less sure of the practicalities of Flooing, though. Quidditch Through the Ages mentions that transcontinental Apparition is very risky and should be attempted only by the most skilled wizards. Pottermore discusses Portkeys, but, again, Portkeys are an imperfect form of magic. They are highly sensitive objects, and wizards using Portkeys are often plagued with motion sickness after using a Portkey.
So, canonically, I'm voting for FLIGHT.