I'm going to go with the seemingly obligatory Beach/Swimsuit episode, where we get to see our characters behave in scanty clothing for dubious plot reasons. The alternative to this, and serving similar purposes, is the Onsen/Hot-springs resort episode, again with the characters scantily clad -- or more accurately, buck naked except for conveniently placed wisps of steam. From either of these special episodes, all sorts of fan-service usually ensues, and often serves to raise whatever sexual tension might be arising between the show's characters.
Also, huge bonus points go to any anime series that manages to squeeze both a swimsuit episode
AND an onsen episode in the same season.
