I have a good friend, a songwriter, who was literally a one-hit wonder. Had a #1 in the mid-80s and never got another cut. He spent the next two decades trying to write something better than that one song, basically hoping to beat it. I'll pass along what he said: "My biggest mistake was living my career in the rearview mirror. I wasted years looking back instead of looking forward, towards the next song."

You aren't in competition with yourself. And your songs are in competition with each other. Every one is different, a unique entity. Forget that one and move on, you've already written it. And also, fwiw, songwriters are generally the worst at determining which of their own songs is their best. Case in point, I had a song recorded by an artist named Nanci Griffith back in the late 80s. Was a single, but died in the 20s or 30s, I don't remember exactly. But a lot of people have said over the years how much they liked the song, or that "it's the best thing you've ever written". To me, it was just an exercise in writing....took me about 40 minutes to write start to finish. I never thought it was special at all, but other people seem to. Conversely, I have a half dozen or so songs that I would describe as "the best I've written", and most of them have never seen the light of day.

Co-writing, I agree, is also a great way to break out of a rut.

Best of luck with it.
RB

http://www.rogerbrownsongs.com
http://www.nashvillesongwriters.com/roger-brown