I have three favorite HM slaves this gen. Two of them have undoubtedly already been mentioned repeatedly - Linoone and Tropius. They're the perfect HM slave pair. But my other favorite, who joins the team late in the game, after all the rest of the content has been dealt with and the rest of the moves aren't necessary, is Parasect. It learns Cut and Rock Smash - the two most useless-but-often-necessary HMs - plus False Slash and Spore, which makes it a perfect dual-purpose HM slave/Catcher.