I'm guessing either Axton, Salvador, or Mechromancer. I've only played as Siren (Lv. 26), and I'm playing with my friend as Gunzerker (Lv. 9). So I don't have a great feel for the other characters. Maya is great for one-on-one fights if the enemy is susceptible to phaselock.
Salvador with the right weapons and mods. Seriously, shock shotgun plus explosive or fire shotgun,then gunzerking, makes dueling everyone a joke. And I am a level 23 gunzerker. For all out damage,obviously gunzerker is the choice. But since you are asking what character makes the game more easy, it'd be the siren.