It really felt that only with Darla he got to be both sides of his Angelus and Angel versions... it made it so intriguing.

Like I'll always remember Angelus telling Darla, "I'll go through the hell of depths to lay by your side." (paraphrasing the quote as I remember it. lol Sure I got some part of it wrong) and that's pretty much what Angel did... he went through everyone just to find Darla again.