Merthur

Merthur is the slash ship between Merlin and Arthur Pendragon from the BBC Merlin fandom.

Fanon
On AO3 it is the most written ship for both characters. It is the most written ship in the BBC Merlin tag.

Fandom
AO3

Trivia

 * Although both characters appear in Once Upon A Time, many people do not ship them in that series, due to their limited interactions, and Arthur being evil in the series.