At the end of the anime, it is implied that Haruhi will be with Tamaki. However, Kyouya's father states that he wants her as his son's wife. And, of course, we can't rule out Hikaru. If there were a part two, with whom do you suppose Haruhi would ultimately end up in a permanent relationship?
Haruhi should end up with Tamaki. Because it's so selfish that the dad of Kyouya can make them get married.
Um, Tamaki sempaimost probably. Well since they both had feelings for each other. Tamaki obviously loves her and Haruhi, well, she does like him. Not as much as the way he likes her but she does has some feeling for him.
But then there's Hikaru as well and she seemed pretty close to him too. She might like him too because well, i think Hikaru might confessed to her that he had feelings to her a lot faster than Tamaki might because Hikaru has the tendency to 'spill' words that he normally wouldn't have said when he's jealous of something. So, he might have accidentally confessed (some day) to Haruhi out of jealousy of her being to close with other guys besides the hosts.
Haruhi should be together with Tamaki, I'd be mad if it'd end any other way, I'd really be... I waited for them to move on with their personal realtionship for all the series... I think the writers know that... they aimed to this all the time...
They put Haruhi with Hikaru too, it seemed like they could have a relationship now didn't it? But the writers fixed that at the ending by saying that Hikaru would be too stupid to fall in love (Kaoru said smth. like that)... The writers want Haruhi to end up with Tamaki as much as I do...
My recommended fanfic: "Dreamer" by Scourge
What if I said Hikaru?
I mean, I can't really see Tamaki x Haruhi or Kyoya x Haruhi going anywhere. Hikaru seems like a better partner. They get along well, too.