I've played on the Prexus, Stromm, Rathe, and Antonicus Bayle servers, and it seems to me that I recall that the most peeps being online at any given time was on the Stromm server. I cannot say exactly what populations are on what servers, but I know that Stromm had a lot of people on at any given time ('cept early morning hours between 3-5 AM). The plus side is that buffs and such are usually obtainable with minimal effort with that many people in PoK at once. Also, Stromm is considered the "newbie" server, so a lot more tolerance (and help) is given to those who /ooc with tons of questions.
Downside? More people means (usually) more competition for camps and a lot more traffic in popular zones (Veksar is crazy right now). Another downside is that bazaar prices are outrageous. And, because it IS a newbie server, you're more than likely to see some really poor players and be hit up by the inevitable, "Hiya...do you have any money or items that I could have?"