The main RK pairings are usually KenshinxKaoru, SanosukexMegumi, YahikoxTsubame, AoshixMisao, Battousai (Kenshin's assassin name)xTomoe or Akira (the guy Battousai killed)xTomoe, SaitouxTokio.
In yaoi, there is usually SaitouxSanosuke, SanosukexKenshin (or vice versa), HikoxKenshin, AoshixKensin (or vice versa).