Juliet for obvious reasons: Elizabeth Mitchell is perfect.
Kate is probably my favorite character though. She's so strong yet so vulnerable and so human. She's not some easy woman who can be told in one page; it takes a whole six seasons to tell her story.
The best relationship is (obviously) Jate. I honestly cry every time I see them together, be it in a gif, a YouTube video, or in my memory.

This sums up their entire relationship:
"I've missed you so much."