Good question. I was hesitating between Sombra and Lightning Dust, but since Sombra has almost no depth to his character, Lightning Dust wins in my opinion. Her character is the anti-thesis of Rainbow's being. She shows that without responsibility, your efforts of being the best can be destructive. She also contributes to RD's character development and forces her to give up her own ultimate dream just to be with her friends, which is a major sacrefice, since Dashie's dream of becoming a Wonderbolt had been established in the FIRST EPISODE!!! Also I have this headcanon where Lightning Dust teams up with Gilda and they become wicked, vindictive antagonists and come up with a plan of settling the score with Rainbow Dash.

She is AWESOME!!!!