Take some time to look around, and see what makes sense for you, imo. ironforge.pro gives some population data based on raid logs, so may not be super representative of leveling populations or non-raiding populations, which may vary from server to server.
If you enjoy Dungeon Leveling (i.e. running dungeons with guildies at level), we have a community formed around just that on Faerlina Horde. You’re welcome to come check us out:
If that’s not something you’re interested in, I do recommend that wherever you level, you find a guild (or create one, but it’s often easier if one already exists) where everyone can invite. Be active, and invite people you meet who play around the same days and times as you do. Keep in mind that if you invite 10 people, you might be lucky if 1 reaches max-level, so the more the better.
If you’re social, and active, then by the time you reach max-level, you should have a good group of buddies to gear up with, and maybe even find yourselves in the same raiding guild (or form one!).
Best of luck out there!