-Houndour: Houndour looks excellent in S rank. Thanks to its amazing mixed stats, great offensive type, and wide move pool it has easily become one of the top Pokemon in LC UU. Houndour serves as a revenge killer, pursuit trapper, wall breaker, boosting sweeper (if nasty plot or flame charge) , lure (hp grass or ground), and it can do all this just by switching up a few moves. (It also stops Riolu sweeps with Sucker Punch if you're willing to play the 50/50 game) Dour is a stellar Pokemon in LC UU and should move up to reflect that.
-Skrelp: I wouldn't mind seeing Skrelp in S rank either. As a poison type it is one of the very few Pokemon that can switch into Riolu and force it out with a powerful Hydro Pump or Sludge Wave. Nothing can switch in on Skrelp, as Frillish, Tenta, and opposing Skrelp both are hit hard by Tbolt. Its speed lets it down in some situations, but its huge bulk lets it beat things like Shrew and the birds 1v1.
-Riolu: I don't believe Riolu should be moved to S because of its small amount of setup opportunity and the presence of other priority attacks. Riolu's mediocre bulk only allows it to set up on Pokemon that can't do much back, like Stunky or Lileep or something. (Though Stunky can have taunt so it can prevent copycat :O) In order to sweep, it needs not only the removal of Skrelp but also ghosts, as crunch is not strong enough to sweep and switching from HJK to Crunch lets your opponent attack or burn you. After set up, Riolu can still be dealt with by using priority attacks like Fake Out, Sucker Punch, or Quick Attack to give Riolu a much weaker (or ineffective) move to copy, and killing it while it tries to HJK again. (Although it is a 50/50 unless FO) Riolu is still an incredible mon in LC UU capable of revenge killing and easily sweeping teams when given proper support, but it is not worthy of S imo.