S-tier starter picks: Dragonknight and Spellweaver. Dragonknight is forgiving for melee progression, while Spellweaver gives clear ranged damage and control.
A-tier flexible picks: Ranger and Steam Mechanicus. Both can be strong, but they ask more from positioning, gear and preferred playstyle.
This list is intentionally conservative. Drakensang Online progression depends heavily on gear, event rewards and upgrades, so the best class is the one you can farm consistently.