Idk if this is the "most used", but I like running at least 92 spd Adamant on Azumarill which lets you get the jump on min speed Sylveon, Clefable, Tyranitar - this lets you bulk up your HP with 160...Max speed Adamant sacrifices some bulk to let you outrun 0 speed Scizor, M-Scizor, M-Venusaur, Heatran, Skarmory, Mandibuzz, Heracross, and also slower variants of M-Mawile and Azumarill.
In the on-sight preview analysis it lists 28 spd adamant which I think is to outrun Sassy T-Tar, but I like being able to at least outspeed Clef too (and neutral nature 0 spd T-Tar if that exists). Happy Azumarilling :)
Oh, as to Rotom-W, they only have to invest 44 spd to even outrun Jolly Azu, so I don't like dealing with that - if you're really paranoid about trying to catch everything possible, then Max Speed Jolly could catch some slower rotoms....Idk if it's worth losing out on the power and bulk tho?