As the others have said, all of them are fine. I usually don’t recommend Krieg to people new to the series because a little experience and understanding of the game’s mechanics is certainly useful for playing him effectively (which in this case means, not getting yourself killed all the time). Since you’ve played BL1 though, I bet you’ll understand him faster than total newcomers. But still he’s a bit tricky with a suicidal action skill and many skills that damage himself. Still great fun being a burning psycho setting everyone on fire before hacking them to pieces.
Zer0 is different to Mordecai, he’s a sniper but also stealth and melee. As others have said, not the easiest character, needs a little more playing skill than most other classes imo. On the other hand, his action skill can be used as an emergency exit similar to Lilith’s Phasewalk.
Axton is basically Roland. Balanced character with relatively straightforward skills, so pretty easy to understand. Good for beginners.
Maya shares some skills with Lilith (like elemental boosts) but is very different because of Phaselock which has many uses. Good for beginners as well, and also excellent in coop.
Sal if you want to go in all guns blazing with regenerating ammo and such.
Gaige… actually pretty easy in the early game after unlocking Deathtrap, but yeah she’s a little odd. She’s got a skill tree that reduces accuracy, she’s got a skill that lets you just shoot at the ground because bullets ricochet towards enemies… her action skill is of a fire and forget nature–just press the button once and Deathtrap will do the rest on its own. Most other classes require more tactical/skilful use of their action skill. My main issue with Gaige is that Deathtrap is glitched and sometimes just stops fighting when you really don’t want that to happen.