Agnar is highly popular right now, because every "New" TLP is filled to the brim. It will die out some, but I suspect it's population will be fine for quite some time even once the "Krono Farm Rush" ends.
Phinigal will probably have the largest population Long term. Easily as popular as the most popular live servers for quite some time.
Ragefire is a bit less than the above, but plenty populated. You won't travel anywhere in PoP (the current expansion) and be alone in a zone.
All I know about Lockjaw, is it's simply a less populated version of Ragefire. Same expansion, same rule set, less players. This isn't necessarily a bad thing, it depends on what you like. It has raid guilds raiding top end content, so it's certainly not "Dead".
Late August Ldon opens up on Ragefire/Lockjaw. Then 3 more months or so of PoP until GoD unlocks. From there on out it's 3 months per expansion (3 months after the expansion is beaten, at any rate).
Ragefire Currently has 2 Guilds in PoTime, another about to be in PoTime, and 2 or 3 more about to enter the Elemental planes. It's likely 5 guilds at least will be in PoTime before GoD launches. It's pretty healthy. If you like Boxing, or using MQ2, I would absolutely recommend Ragefire
The top guilds all get along and rotate. They give lower tier guilds opportunities to enter into the rotations, and have worked together to block a few guilds who have refused to rotate with the lower tier guilds. It's a pretty good situation on ragefire
