It depends by what you mean by need. If it is for his career as a detective, definitely Ai. I can't see Shinichi as a detective if he ends up with Ran.
Say, he needs to go on somewhere far for a case. Ran will worry her guts out and thus reducing the time Shinichi could've used for deducting by phone calls and dealing with her wrath (and worry). Even in a local job, things can get pretty heated up. In the worst case, Shinichi would abandon his detective job to not worry Ran too much and supress his curiosity and talent.
In Ai's case though, I couldn't see her fussing over Shinichi. If any, she'll be helping him. Like Holmes to Watson. Or Irene to Holmes. They don't necessarily have to be married, but I guess getting together will be make some things easier to deal with (like, fangirls, shippers and Ran) and will make things much more interesting.
I welcome any counter arguments
