probably the washing machine if I had to guess, do state why it is most viable eg movesets type matchups etc
Rotom wash is the best and most used. This is mostly because of its great typing and ability combined with solid defensive stats allowing it to tank a few hits and potential burn something or do some solid damage thanks to its respectable special attack. It like all the other rotom forms is still not great in the current meta because other Pokemon out class them but if you want to use a rotom wash is best.
Wash is also the only rotom form that doesnt get instantly destroyed by one of the top Pokemon so that helps its case as well.
Source: Experience