As you are a new player, don't listen to these old farts above, none of them remember what it's like to be new apparently.
The nation where you'll have
the most fun is Yllse, there is no doubt in this, but there are also many silly people here who you could have inner nation problems with, mainly the Brazilian and Argentinian players. Yllse is also the fastest for leveling from 1 to dual class 1. As a newbie, there are some awesome folk always around to help, like for the time being, The Syndicate guild.
Zypern is a nation of bloated egos. They always think and claim to be better than they really are. I guess they're fine to some extent, but for the most part they seem the same as Yllse in that aspect, except they're less active and they have less lower levels. You will have
less fun there. Positive notes: You might possibly have an easier time leveling there when dual classed, but getting to dual class will be a pain in the first place. There are also less inner nation arguments than in Yllse, so you'll avoid drama.
Vyseus consists mainly of 2 mafia guilds and a Taiwanese/Chinese/whatever guild. They're quite inactive as I see it, you will get barely any help leveling if any. The bright side is, Vyseus has the most stern people, you will find the least amount of idiots there and the least amount of drama, but you will also definitely have
the least fun there.
As you can see, every nation has its ups and downs, I wish you good luck on making a choice. If you're down for a hardcore, but calm path, go Vyseus. If you're down for a fast way to DC1-4 on a very rocky mountainous road, go Yllse. If you want to take the middle path, go Zypern.
EDIT: Just noticed you're not actually a new player. Nonetheless I will save this post for future use.