List of strange pokemon used by world champions: empoleon, escavalier, hariyama, gothitelle, bronzong, toxicroak, smeargle, snorlax

List of runners up: shedinja, moltres, yanmega, eelektross, haxorus, krookodile, exeggutor

This list doesn't include junior divisions, or senior divisions for 2011-2013 (masters division was created in 2011, before that seniors was the highest.) I also probably missed a few, since I couldn't find the runner up for 2008.