Bingowings said:

As posted earlier if Darth isn't a name but an assumed title Obi-Wan using it becomes a dig.

Like Han's "Buddy" and Mace's "My Lord".

Even as an assumed name it's still a dig.

He isn't "Anakin" and he isn't "Lord Vader", he is "Darth".

As friend or worthy foe he would command respect but as the turncoat traitor Kenobi sees him as not deserving recognition of rank (even an enemy rank).

Nah. He would have called him "Anakin", just as Dumbledore called Voldemort "Tom" when they dueled in Harry Potter 5.

From my limited knowledge of EU, "Darth" is a title implying some sort of worthiness, and of which the bearer is proud. Interchangeable with "Lord" if i'm not mistaken.

So, you have this dude who thinks he's now a badass and has taken on a new name to go along with his new persona. It's quite an insult to not acknowledge the new name. So.... "Tom" or "Anakin".