They both have their ups and downs, but I would say Zoro. Sanji is lebih of a ladies man and I think it would be difficult for him to stay in a single relationship even if it would be possible for him to stay faithful. Plus, his nosebleed problems would be bothersome. Zoro over-trains which makes me think that he probably won't be as romantic as Sanji, but that is completely fine with me. He may get Lost easily, making it just as bothersome as Sanji's nosebleed problems, but it would be fun to guide Zoro and try to get him to stay on the correct route.
definitely Sanji... I mean I like Zoro too, but he's too ignorant and it would be hard to get his attention.. But Sanji would be a perfect boyfriend, he's an excellent cook, perfect bodyguard, and he'll be around 24/7... and he serves u well!