For the ask meme: Arrow
Easy: John Diggle.
John Diggle is, without question, one of the best original characters ever introduced into a comics adaptation besides Harley Quinn. This is for several key reasons:
He's Little John. Now, Green Arrow plays fast and loose with its Robin Hood parallels, in large part because initially the Robin Hood aesthetic was just that, an aesthetic, but in the Arrowfam of the comics you don't really have a Little John variant - all of the Arrowfam members hew closer to other Robin Hood characters, Ollie doesn't really have a 'close friend who's loyal to him but unafraid to tell him when he's being a moron' - the closest you get is Hal, but even then the parallel isn't as 1-1 as it is with Diggle.
He is Ollie's close friend who believes in his mission but is unafraid to call him out on his shit. That's a key part of any member of the Arrowfam - Dinah, Roy, Mia, even Connor, all of them are defined as much by their antagonism with Ollie as their kinship. Green Arrow stories are, at their best, about a flawed man trying desperately to be better, and Arrow often shits the bed with that by making people too forgiving and accepting of Oliver's shit - but Diggle is often the exception.
Everything about his introduction makes sense, and he never feels like he's replacing another character. The reason Diggle is the character from Arrow to get ported over to the comics and stick around through various different writers and status quos is because there's not really anyone in the Green Arrow side of the DCU like Diggle. His backstory, at least as portrayed in the first few seasons, manages to slot neatly into a whole load of the characters without feeling like the world is twisting around him, unlike some other characters I could name. He's a bodyguard hired by Oliver's mum to protect him, so that's Oliver. He's got a dead brother who was killed by Deadshot - makes sense, and it's not like Deadshot would remember or even really care about one specific target - It Was Tuesday is in full effect here. John Diggle is that rarest of characters - one who slips neatly in to the point that if you don't know he's from Arrow, you'd assume he was always there.
Now, you may have noticed - everything that makes John Diggle great is dependent on ignoring the stupid (and, though it's not my place to say it, kinda racist?) fan theory that somehow became canon that he's actually John Stewart. If you sidestep that, John Diggle's unquestionably the breakout hit from Arrow, and the one shining nugget of gold in Arrow's steaming pile of-
