For me, IMO, both Shinichi's parents, Kid, and even Haibara (mainly for that case in volume 21) have shown to be able to consistently solve the case in case the two main detectives are elsewhere. (Albeit, I only counted Kid because he always sees through the plan of the police.)
It is between Hakuba, Akai, Kid, and Yasuku. But I have to give it up to Hakuba, he's a rare kind....and not related to Shinichi's make-up in any way....
Hakuba I would say. Yeah Yuusaku is actually better, but I don't think it's really fair to count him because the only times he pops up, he's just there to say "oh I knew it all along I'm glad you finally figured it out!", so he's sort of a cheater.
If it has to be "detective" literally, I´d say Yuusaku, if not, then because of his cunningness (does this word exist?) etc etc as we saw in the BO arcs, Gin.