Having played on all of these servers i would say that
For premades:
Horde: AP is larger than BWL but the quality of players is generally the same you just have to know who to look for. On AP it will be easier to find a premade at all hours, but not always with those specific people you are looking for. BWL may not have activity 24/7 but when they do you'll almost always have quality people to play with
Ally: This one is similar to the horde, except i would say that AP=Vashj>MG in size
For chat:
All servers have there great chat moments. All servers have their share of douchebags that want to ruin your fun. AP just has more of the latter because it has more people. On AP it might be harder to have a small conversation in chat because so many others are using it. While on other servers it might be hard to get anyone to talk to at certain hours, activity is all based on the times that you can play. '
What servers i prefer:
Having played on them all and having 20's on them all as well as extensive experience being the only person on a server I would put them in this order based on personal preferance alone, this is how i like them not how the quality of the server is.
For ally: MG>Vashj=AP>Alone
For Horde BWL>AP=Alone
Biggest tip i can give you is on any server find 1-3 friends that are on when you are and play with them, You may ever end up on a server I didn't say. Some people like the quite life, and some people like to play with lots of others. As for AP its a love it or hate it place you just have to play on it yourself to find out, dont think that everything said here on TI is 100% true and un-biased, because it isn't. Every post made on here, even mine has a certain level of personal biases. No one can tell you which sever is best for you. That is something that can only be done through actually rolling a toon on that server and playing there,