If you count legendaries/sub-legendaries and forms or regional forms, then Zacian-Crowned is the fastest Steel, at 148 Spe.
If you count legendaries/sub-legendaries but not forms, then Kartana/Durant at 109 Spe. If regional forms count, Alolan Dugtrio at 110 Spe.
If you don't count legendaries/sub-legendaries nor forms or regional forms, then Durant at 109 Spe.
Hope I helped!
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