Gotta Go Fast

For reference, all tests are done with one of these 2 builds:
Reinforced Greaves, Hastened Fatalis, Winged Blade, Witchblade, Heartseeker, Golden Bow
Reinforced Greaves, Hastened Fatalis, Winged Blade, Witchblade, Rod of Asclepius, ---
Always carrying the speed buff, and values are taken with Greater Sprint active
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Now, I'm not sure if the game is limiting these gods to the listed value, or if the game is only displaying the listed value and they're actually travelling faster, but the "speed" of 754 seems to be the cap for most gods. I'm going to split our gods into 3 categories: Those who can't reach 754, those who can reach 754, and those who can exceed 754. I'm not going to list who can't because that's literally just "everyone else", and reach 754 is of course inclusive of everyone who can exceed 754.
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Capable of reaching 754

Vamana, Sun Wukong, Geb, and Scylla.

One of these things is unlike the others. Most obviously is that Scylla is a girl and the others aren't, but also because Scylla achieves 754 through just numbers. Vamana has his Clear the Path, Sun Wukong has his Bird, and Geb has his Rollout, All of which take away some of your control and just make you into sort of a vehicle that you steer. Scylla on the other hand gets a 70% speed boost from her ult, which I believe is the highest in the game, which combined with the speedster builds, speedbuff, and greater sprint, put her at the 754 "cap"
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Capable of exceeding 754

Thanatos

Thanatos' Scent of Death might only give Thanatos a 30% speed buff, but that value doubles when he is moving towards a target within his execute threshold. Furthermore, his ult gives him an additional 150% speed, bringing his (displayed) total to an impressive 882, though you'll never be able to tell because you're flying up high in the sky and can't appreciate just how fast you're moving. - http://a.pomf.se/xsqcyn.webm
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Guan Yu

Interestingly, Cavalry Charge doesn't display any change to your speed, but you can feel the difference, as well as outrun other people so it's kinda obvious you are in fact moving faster. In the following webm, Thanatos (travelling at 824) and Guan Yu cover about the same distance. Thanatos taking ~5s and Guan Yu taking ~4s, which makes Guan Yu the fastest god in the game, capable of reaching ~1030 (though the item window says the cap is 1030). Comparison - http://a.pomf.se/nveoef.webm
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I've been ignoring Mercury because using the "Fastest God Alive" feels like cheating, but since he doesn't actually have any movement speed skills he's just fast in the sense that his average walking speed is faster than everyone else's. Although if you consider Sonic Boom to "count", he has a fixed travel time of 0.2s, which at maximum distance makes his speed in that 0.2s something like >11k