Amell has a natural charm and charisma to him. Awkward acting style, but he has charm. Just watch season 1, or his Facebook videos.
With that said, to say that Olicity has better chemistry than Laurel and Oliver is kind of a one sided argument. Again, the reason why so many people fell in love with Felicity is because of her quirky attitude that comes off as very cute and funny, even in awkward moments. That's not Olicity chemistry, that is EBR's acting carrying that. The fact that she had even more chemistry with Ray and Barry, two actors that she had less time working with, showing how much of a factor she is on her own. All of Felicity scenes were great in the first two seasons, until the romance was introduced and the character was written horribly and lost all of that spunk until this season. But last night's episode reverted her back to what didn't work last year.
As far as KC is concerned, it was like night and day watching her small appearances on Legends and Flash, and seeing just how much more likable she was there. She definitely has the ability to be likable. The reason why she's not on Arrow is up to speculation, but it's ironic how the guy in charge, Guggenheim, was not involved in those other two shows as he is not the showrunner, and Laurel was able to be herself and be more likable in the process. The fact that Laurel is not allowed to have an arc of her own that doesn't involve Sara is just one of many issues that keep her from being able to win over some fans.