I play on Oasis, and there always seems to be a "medium" population.
I guess it depends on what server you wish to play on, and what time of day you intend to play. My standard advice is this: When you make a toon, make it at the time you intend to play, and then choose a server that is either light, medium, or high population, depending on what you like. Sounds like you would like a high population...
Keep in mind, though, that this game is what, 4 years old, now? Maybe more. Therefore, most of the population on any server will be close to, if not, max level, so there will not be many opportunities to group at lower levels unless you get someone willing to mentor.
Now, that said, welcome back, and if you come to Oasis, send me a tell (Aubalys), and I'll either mentor, or dust off a lower level toon...