I'd like to join in on the "_ fell first, _ fell harder" debate because I have been seeing many posts which argue that Merlin fell first and Arthur fell harder and... Ah ah ah ah. No. No that is not what happened.
Arthur fell the moment this twink with an attitude came to him and called his ass out. No one aside from Morgana has ever dared SASS him in his life and it made him find out some things about himself. Whereas Merlin saw an asshole and said: "What a bitch" and tried to punch him. Arthur liked that. Merlin didn't like Arthur sending him to the dungeons.
In the first episodes, Merlin is keeping him safe because of destiny, while Arthur is willing to trust him (ep. 2) save his life after a magic confession (ep. 3), and give his life (ep.4).
Arthur fell first.
By the time season five arrives Merlin is lost. He hasn't just fallen for Arthur. He has dug himself a hole so deep he can't see the bottom just that he can fall that much harder. He is quite simply obsessed with keeping Arthur safe and happy. Not for his destiny (which he has by now completely forgotten) but for Arthur.
There is no one else in his mind but his King, so much so that in the first episode of the last season he would not think twice before leaving all his knight friends to die, if it meant bringing Arthur to Camelot safe and away for Morgana. He gives up his and thousands of people's life and freedom for Arthur. This guy is completely gone for Arthur and can't think about anyone else.
On the other hand, Arthur CAN and does think about others... Gwen, his knights, and his people, even if he values Merlin "above all others".
So Arthur fell first. But Merlin fell harder.
Dat's how I see it anyway, feel free to disagree 💪