Sarah of course... there's so much tension between them!!

Since Jill moved to the big house up the river the writers answered your question for you. Besides Sarah and Chuck have been in love since they kissed just before finding the bomb was really Brice Larkin way back. Now that Chck saw Sarah kill the Fulcrum dude I think Casey will set him straight on that issue and the tension between Chuck and Sarah can begin to build a new.
That's my thought and I'm sticking to it.

SARAH !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Since Chuck only liked Jill and she's a criminal on the run and he Loves Sarah and she him as we now know it's a moot question at this poimt.

please not this I love Charah but also like Jill hate people bashing making her the badguy or just a mean girl.