Thanks to Cassini, we got spoiled for flybys over planets and moons – just think that we are now at number 111 (the last one was carried out on May, 7 over Titan’s Xanadu region – see this link for the footage).
More it’s to come, from Cassini itself 
(the next one is a Dione Flyby. On Jun 16 2015, Cassini will pass at 517 km – 321 mi – from the small Saturn’s moon and hopefully snap a few good shots) and from other probes, first of all New Horizons with its Pluto’s encounter July, 14 2015.
In the Summer of 1989 – on August, 25 – Voyager 2, after having visited the two gas giants and Uranus, traveled at just 3,000 miles above Neptune’s north pole, its closest approach ever to a planet during its 12-year run through the Solar System. Simply amazing.
As this was not enough of an achievement, a few hours later the spacecraft passed about 25,000 miles from Neptune’s largest moon, Triton, the last solid body visited on that journey (so far, but you never know).
Below is a video of that spectacular encounter:
There were, of course, a lot of other candidates for the place of the best flyby. What’s your chosen one?