I may be a little annoying (which I know is brave, seeing it's my first post here), but I don't consider any Brock replacement worst then Brock himself. Brock is definitely my least favourite Pokémon Anime character. His womaniser joke was annoying from the very start and definitely uncalled for in children show. He had little to no character development and the same goes for his Pokémon too (to these days I consider the worst thing writers did was giving Brock Mudkip, a Hoenn starter, which thanks to this, got no proper screen time and character development). He was simply just there, feeding the others and (in OS) preventing Ash and Misty to kill each other. And then, when they decided they want to finally get rid of him, they will, once again, come with Brock, who spent five sagas bragging about how he wants to be the best breeder, miraculously turning into Pokémon Doctor wannabe. That was bad end for bad character.
As for his replacements, Tracey was OK. He too had little to none character development, but that is actually quite acceptable, considering his short time as main character. He was quite competent and he could help a lot. But his leave from main cast was also quite out of character, as he was shown to love traveling and exploring new places, only to end being locked in Oak's lab. Yes Pokémon Anime writers did that more time after that (Dawn's Ambipom anyone?), but Tracey's was actually first, so it somehow stuck in me.
Cilan, well, he was actually what Brock was in OS, coocking, taking care of Pokémon and preventing Ash and Iris to kill each others. He also got no real character development and neither did his Pokémon. What put him little above Brock IMO is that he has no inappropriate and annoying habit similar to Brock's girl hunting. But, to be honest, the writers tried too hard to get the show back to its roots with Ash being one incompetent main character (who even started to do rookie mistakes, something we did not really see since AG) and two sidekicks, that it started to get old really quick. Hence the whole Unova and Decolora are actually quite forgetable sagas, prety much like Johto was.
Clemont... well, yes, he is coock and Gym Leader, but he is fighter and has a lot of character development. Hence I am not sure if he really fits the definition of Brock replacement. For me, Bonnie more fits that definition in XY, by taking care of others Pokémon and being annoying with her marriage propositions for her brother. But even she has some character development and I hope she will have more in XY&Z. Serena is the one I worry most now, but that's for another debate.