I'm just going to flip this over to Pokemon Voting Polls, as this is more of a question and answer thread than a discussion one.
While it's hard to gauge these things based on Pokedex entries, it seems as though Exploud would be the loudest Pokemon. It's said to have cries that can be heard over six miles away (putting it in the same category as a smallish clap of thunder), but it is also said to be able to create earthquakes with its voice, which means it should be much more powerful than that. I don't quite know the decibel scale, but I'm sure that hearing an Exploud's cries would be like standing next to a fighter jet taking off with that type of noise. I'm sure many of the legendary and other large Pokemon would be quite nosy, too, but they might be closer to a lion or elephant in terms of they're cries, rather than having eardrum shattering ones.