You mean, NOT one we'd pick because of an emotional attachment, but one on its own merits outside of that? I suppose quite a few people would have trouble with that as their 'best ever' song might be BECAUSE it has an emotional dimension.
Penny Lane is for me in many ways a perfect pop song. It does a thing I love in music - musically it's going along very bright and catchy in the verse then makes that slight unexpected turn (e.g. just after "...pleasure to have known..."), and the song as a whole creates an entire picture in my head. McCartney seemed to brim with this stuff, it was as natural as breathing, and though the word has long been over-used to death I'd suggest it's a kind of genius. It's partly why I've always resisted the "Lennon was the cool one" lobby, and why I'm not quick to join in condemning Macca for his most recent under-par performances. I'm always inhibited by thinking "Yeah, but he was in the Beatles and wrote Penny Lane, and that".
The Carpet Crawlers by Genesis could be my favourite rock song, really atmospheric and with a hint of tension. And was apparently knocked off by Peter Gabriel in a hurry because he was late and there was a gap in the album. It shows they weren't the noodling prog band they're sometimes painted as and could quickly come up with a 5-minute gem. Planet Telex by Radiohead comes close too, I'm a huge fan of theirs and that one was what made me a fan, a bit like The Wall making ummmm a Floyd fan. It has pretty much every dynamic and texture I love in rock music. (Possible other thread: what song made you a fan of that band?).
I'd have to put in a mention for Glen Campbell's version of Witchita Lineman as well. The music on its own makes me well up - such a haunting quality and beautifully crafted.