I would say that, for a beginner, Paladin might be a difficult choice. They're a very strict class that generally has to be roleplayed a certain way depending on their deity. You have a little less freedom to play them however you want, otherwise you lose access to special Paladin privileges. That said, Paladin's are a good class to play given the amount of Undead and evil enemies you will face on this server, you will definitely have an advantage there. Fighter is probably the most simple class, but they are limited in how much they can develop unless you intend to multiclass.
Unfortunately, I have no idea what the most popular class is on this server. If I had to say, I would guess maybe Clerics (powerful class with healing and buff spells, versatile in combat)? As for least popular, I would say probably Monks (very difficult to level early on, somewhat strict roleplay limits, very fast/powerful in the high levels).