It's hard to say. Most of them have been through some serious shit. Personally, I think Carl has had one of the more troubled runs on this show. Fortunately, he still has family.
He's been shot - went through a very painful experience. Screaming his lungs out as Hershel attempts to take the bullet fragments out, blood-loss, violent seizures.
He was shot again, this time disfiguring his face for the rest of his life. We haven't seen much of him since then but he did make one comment which really made me feel bad for him: "A kid with a messed up face wouldn't be the best first-impression anyway" - he stuttered with a weak smile. He's clearing feeling self-conscious/insecure to some extent. Furthermore, he has to deal with a very real limitation that the others have no idea what it's like to experience. Living with one eye at all let alone in a post-apocalyptic world sounds really rough.
Leading up to his Mothers death he was taking his Fathers side and being rather mean to her. She ended up dying in such a tragic manner. Carl helped deliver his baby sister then shot his Mom in the head.
He's experienced guilt on a level that most characters haven't. He's partly responsible for Dale's death, a pivotal member within the group. That's something he's always going to carry with him. It can't be overstated just how much guilt someone would have for that. It could be argued that the effects of that decision he made are felt even today. For example, recently he was with Enid and he said "Michonne is outside, I'm not leaving it" in reference to the walker. Perhaps this was a subtle call-back to how he left the Dale-Walker.
Attempted rape has been made on him
He's dealt with suicidal thoughts. He basically gave in. When he thought Rick turned into a walker, he couldn't do it, he didn't want to be alone - he'd rather die.
He was forced to shoot his dead Uncle figure [Shane] in the head.
He's either made or tried to make friends around his own age a few times now and they always end up dead. Despite this he still gave Alexandria a chance... just for it to be thrown in his face.
He was forced to grow up way beyond his years and stuff no teenager should be doing. It was quite sad in S5 when he was offered to play video games or watch movies, he was literally speechless.
He's had to deal with a shit-ton of patronizing throughout the seasons, even while he's literally saving peoples lives. The adults are looking out for him but it must infuriate him to be treated as an inferior when in their world its the living and the dead.
He's had to deal with some really... difficult and different situations. For example, when Lori died, Rick was going off the deep end and he wasn't really around for Carl. Daryl, Carol, Beth and the others were doing what Rick should of been doing.