I think the best of TPCi's dub is relatively equal to/sometimes even better than 4kids, though I have to agree the dub has been going downhill after BW. XY was the first series where I actually started watching the subbed version alongside the dub, and the difference in quality is really noticeable. It's not just the music, but the dialogue also learns more and more into just "endless exposition" territory. It's kind of hard to explain since every Pokemon series has a problem with exposition, but there's a certain... lack of personality in (most of) the dialogue. The characters just feel like they're describing everything instead of showing off their character through their lines.
SM has to be the worst dub, though. I've actually watched very little of SM because of how bad the dub is, and if I ever catch up on the series it's definitely gonna be with the subbed version. It's even worse than XY's dub with exposition dumps, and the voice direction is just worse. Even the returning voice actors for SM are much worse than they were in DP/BW/XY.
Also one other small thing about TPCi: idk if anyone else has noticed, but it really bothers me that TPCi will NEVER refer to Meowth by pronouns. They'll always say "Meowth" instead of "he" or "him," even though Meowth is blatantly male and this has been known since the beginning.