Most populated American server
try guesting to servers to see which is popular during your playtimes, however TC and blackgate are pretty popular
I am on Sanctum of Rall and it feels pretty populated. I don’t go there too often, but every time I pass through caledon forrest it end to see people out and about.
Keep this in mind though: You are not tied to that zone at all. You can go out to any of the various racial starting zones, and you are very likely to see a good number of people hanging around in the human one, Queensdale.
Biggest thing I can suggest: join a guild. And by guild I don’t mean some mass of people that you are a faceless pixel in. Seek out a guild that actually plays and does things together. It will improve your experience with this game a thousand fold.