Well, Papi and Suu are actually ranking who's the best at teaching them (though Darling is one of them he's not ranked… I guess he's a given, though), but whether it's about being a teacher or being a waifu… objectively speaking, Rachnera probably is the best. She's smarter, wiser (intelligence and wisdom are two different things, after all), more mature, more understanding, the most caring (though she comes off as aloof), the most careful (she's about the only character in the whole series that's pretty much only ever hurt 1 person on accident, that one girl from her original host family… however, she does "hurt" people intentionally, but with enough control to never seriously injure anyone), she causes no problems for anyone, actually helps solve everyone else's problems (often coming to the rescue unannounced, sometimes without even revealing herself, like with Lilith), and I'm sure the list could go on.

Also I have no idea what "serious issue" you could possibly have with this totally legit and factual waifu ranking…

In the actual popularity poll, Miia placed 2nd. Rachnee still took the first place though. :P

That poll was quite a while ago though… I wonder if there's been another one?