By far

LANA!!!!!!

Clois forever

Clark/Lex is my favourite relationship on the show. Mostly as friends, but their dynamic definitely had romantic undertones at times, and I think it's mainly because of the tension that builds between two friends who are destined to be formidable foes.
Also Clark/Jonathan. This relationship was the stronghold of the first five seasons; Jonathan and Martha are the reasons that Clark is the superhero he is.
Clark/Chloe only as friends. Smallville tended to drive me up the wall when it came to Clark's romantic relationships. It's why the Clark/Chloe dynamic frustrated me in the earlier seasons, and I only really enjoy their dynamic later on. The 12-year-old in me loved Clana, but I don't ship it anymore, mainly because I dislike what that relationship did to Lana- I felt like Clark placed her on a pedestal, and that was unfair to her. The drama between them was also dragged out for far too long. The show wrote Clois' relationship poorly, I felt, and it fell prey to the prolonged will they/won't they which I despise. I like the relationship more in the final season, though.