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I suppose Tartarians are the best : size 6 non-fatiguing undead. I had a lot of fun with a PoD also, as it flies naturally !
There's no point putting boots on Elephants or Behemoths, they already have Trample ability. Boots of Flying would be great though !

Other big units (Titan-size pretenders, Jotun leaders...) are good candidates but beware the fatigue effects, and as tramplers don't use their weapons it's better to give them 2 shields to help defense.