I pick neither. :x
For starters, I kinda doubt that Shinichi loves Ran. ^^' I really do. WHY in the WORLD would you place CASES above the woman you love? XD You wouldn't. You wouldn't be constantly late, you wouldn't not show up, you wouldn't run off in the MIDDLE of the date. < He's done THAT more than once I'm sure! XD You would stay right by her side all the way through if you truly loved her. Shinichi probably wouldn't even BE Conan if he loved Ran, from my point of view. Not to mention, Ran shares no interest or anything in common with Shinichi besides morals. She does karate, he plays soccer and solves cases. She likes romance, he likes mystery. Her father was a cop, his father is a detective and author of mystery. Her mother is a lawyer, his mother is an actress. Her parents don't live in the same home, his are away period.
I don't like the idea of AiCon either. They don't share the same morals, first of all. Haibara, if a man walked into a bank in front of her, robbed it and killed somebody on the way out, would probably simply call the cops and play it safe. CONAN, however, would run RIGHT AFTER that penniless (or now that he robbed a bank very rich) murderer and knock him down. Haibara and Conan would raise their poor child to be confused and misguided. XD They wouldn't know WHAT to do or WHO to listen to if a somebody robbed a bank and killed someone in front of them. Listen to Mommy or listen to Daddy? Hmmm....? That would tear the poor little thing apart! (Unless, of course, he/she already HAS a favorite which would not be a good sign...).
Also- Conan would be reminded of being poisoned, shot, knocked out, the fear he felt and all the lies he had to give and all that he was forced to leave behind whenever he's with her. I don't think he'll ever be able to see her as a romantic partner when she, on some level, RUINED HIS LIFE. XD lol Now, to the third point. Haibara ALSO has very little in common with Conan. Besides soccer and being shrunk and living in fear out of the same organization, they have NOTHING. Haibara, while once temporarily interested in detective work, ultimately chose science. Conan doesn't CARE for science although it is needed to be a detective. If he didn't need it to do his job he'd probably have no interest in it. Once they both got home neither would be all that interested in each other's work. XD
"Hey honey, how was your day?"
"Good. While I was working on that new formula-"
"Hey, sorry to interrupt but I think I'm going to take a shower." Is something how I think it would play out... every day. And then Haibara never brings up work again. XD