There is no best class. All have their advantages and disadvantages.
Eir = Invaluable in a party later on, gods at PvP (Fight a Eir 1v1, and they will probably own you.), however they are pretty bad at PvE (They can still hold up nicely though, just not as well as Sieg or Dainn)
Dainn = Awsome at PvE with all his AoEs and powerful spells, however, I never found him very good in 1v1 PvP, but that's just me.
Sieg = Good at PvE, also good at PvP, and pretty good at owning bosses. Although they tend to take a beating A LOT since their close fighters.
Really all the classes were built for something good and were meant to work together.
Oh and if you want the best class, then its Lime rofl.
He is hax at 50+