Everybody thinks that clerics are the easiest class >__<"
I actually would say fighters.
I never got very far on my cleric, so I guess I wouldn't know a lot. I played a cleric for the joy of being able to heal others and buff my party and kind people. I thought that clerics were too hard to level up with. I nearly got stuck on lvl 16 because I could never get through it. Fighters seem to be able to kill fast and take hits, which would be in a good favour <3
I would say mages are the hardest. The only thing I envy about them is their skills, which are all prettyful.
Archers in my opinion are on the easy side of the spectrum. I'm not very far into my archer, but it seems okay. However, I'm now experiencing difficulty leveling because I don't normally have a party to be with. My friends are almost always in MT and I'm not high leveled enough to be there, and I don't want to reap exp off them. Almost all of the guildies are higher leveled than me. >_< so yeah. I would say that fighters are the easiest though.
By the way, I find your title a little confusing. I would say there is no best beginner class, but there is an easiest for everyone <3 Most people say clerics are the best beginner class, but I found playing an archer much easier. So I think that easier would be a better word.