Fastest Godzilla

I think kotm Godzilla is one of the few Godzillas that have shown to full sprint, although I would give a chance for the famous Godzilla's drop kick. 

The G2K Godzilla could charge at 24.8 mph. Given the GxM Godzilla had the same physical stats, it's likely he had the same speed.
The Heisei Godzilla, at 80m, was faster at 37.3 mph. A source using his 100m height clocks him at 40 mph.
If we're counting the '98 Godzilla, he could reach speeds of 300-500 mph. (Depending on the source.)

The Showa Godzilla's land speed is unknown, but he was able to fly at Mach 1 (767.3 mph) when he used his atomic ray as a rocket booster.

It terms of swimming speed, the Kiryu Saga Godzilla could swim at speeds of 20 knots.
The Monsterverse Godzilla has a swimming speed of 40 knots. The Heisei Godzilla at 80m is a little faster underwater and can reach 45 knots. A source using his 100m height reduces his speed to 40 knots. (Which seems unlikely, but... stats are stats.)
Meanwhile, the Anime Godzilla could swim faster than advanced subs of the 2040s. Given that the fastest known submarine (unofficially) reached 44.7 knots, that would make the anime Godzilla's speed 44.8 at minimum. But given the assumption that fictional submarines of the 2040s were likely much quicker, the Anime Godzilla is probably a great deal faster underwater than the Heisei incarnation.

Fastest overall running speed is definitely Godzilla Junior from Godzilla: The Animated Series. He clocks in at 300mph at max, which makes sense given his streamlined theropod-like design and the appearance of his gait in the series itself.

However, the Monsterverse Godzilla is pretty much even for that title now after KOTM. As revealed by the VFX artists, they had to break physics even harder than normal to get him running to meet Ghidorah in Boston. They cited over 200mph, but calculations done by physics-minded folks have found it's actually closer to around 300mph.

In a sprint, these 2 Godzilla incarnations are the fastest around.