Hey there,

You might want to look at this website to get rough numbers on world populations:

https://ffxivcensus.com/

As you can see, Balmung, Gilgamesh, and Leviathan are the most populous worlds as of September 2017 in NA. Please keep in mind, however, that the numbers are estimate and only counted for players who have progressed far enough in MSQ (i.e. sprouts are overlooked in this census and veteran players who are on hiatus are included).

I'm pretty sure both Balmung and Gilgamesh are congested right now, so Leviathan might be your best bet (it happens to be my homeworld as well, hello there!). However if dungeon queue is your main concern, I don't think it really matters which world you're on because we only have 2 datacenters in NA; aether and primal. Both are roughly equal in terms of population.

Hopefully this information will help you and your friends in the decision-making. Good luck!