Its hard to determine a "best". People have different tastes and styles when it comes to what they want to play. Here are some opinion about the PVE calss in Runes of Magic , hope helpful for you ...
If DPS:
Mage / Priest (Especially if complemented by a Priest/Scout): Mage / priests due to their 15/15 elite buff are able to dish out loads of damage and a decent rate. If you befriend a Priest / Scout, and stack their 15/15 elite, you will be able to tear through most mobs in very few hits and able to duo most elites close to your level.
Scout / Mage : Even though wind arrows got nerfed, most of the other skills actually recieved a buff. Getting Speed Shot Mastery raised when available and liberal use of snipe, combo shot and shot in conjunction with your rooting skills and using wind arrows just to get the frost effect, you shouldn't face too much problem. Though Cost of Arrows is a bit much ._.
Priest / Scout : Although people often hate "runes of magic gold" this class allow's you to play priest and actually dish out comparable damage to a mage. 15/15 elite skill gives you the best magic attack buff which boosts your damage a great deal. Rising Tide isn't useless, keeping RT up and your passive for water damage/res and 30/30 elite skill will boost this skill greatly. The new system for DoTs in combination with your 15/15 magic buff makes DoTs a very valuable skill to have. It wouldn't be uncommon to see high 2xx to 4xx damage (and even higher in some cases depending on magic defense) per tic on mobs with bone chill. Vamp Arrow isn't As useful anymore but it can still add that tipper when dotting mobs.
Tank:
Knight / Warrior: Arguably the best tank in game. You have the highest HP and Defense in the game, and not to mention the damage isn't all that bad. 15/15 boosts your aoe's allowing you to effectively AoE grind with little problem. The changes to the rage system make getting rage easier to get and keep(rom gold), making it easier to use slash and whirlwind. You also have defense formation, which you can pop if you need that extra defense for whatever reason (I only really used it as a life saver when solo to pop up my hp easier, and when the priest isn't able to heal good enough to keep my hp up to a safe level)
Knight / Priest: Another good choice. Able to Mitigate damage well and able to keep alive in some tough situations. Easy to solo, but can be time consuming and is quite the mana chugger. Nice DoT at 15/15. Regen, though nerfed can still be useful in this situation. (This was my first char in RoM, and though i liked it,you can buy runes of magic gold, i probably could have built it better so getting opinion from someone else would probably suit anyone reading this better)
