Pretty much all 18s use one of the following three pairs of boots:
Cow Kings
Treads of Cthon
Rare Greaves (for a kicker) with 20frw, 30+ fire res, 30+ lightning res (nice adds are 3dex & 10fhr)
At 30 you have a bit more flexibility:
Rares (type depends on class/build/other gear - almost every base is viable for some char): 30frw dual res (cold>light>fire) with at least one of the following [10fhr, third res (not poison), 10-20 mana, 4-5 dex, HFD, Poison length reduced]
Lots of sets and uniques have some use too: Cow Kings, Infernos (Europe only), Tearhaunch, Sanders (used but arguably shouldn't be as you can't str bug them), Nataltya's boots, Goblin Toe