If we’re talking solo, “Jealous Guy” by John Lennon. To be honest, 99.999% of the post Beatles output by solo Beatles did very little for me…John’s music was the best of any of them, but I think the Lennon/McCartney songwriting was far greater than the sum of its parts…they balanced each other in exactly the right ways. I think it was a case of John being all substance and Paul being all form…I find all of McCartney’s output to be just way too simple and poppy for my tastes, and though Lennon’s work was powerful, it didn’t have that hook, that last level of varnish if you will that Paul brought to the mix.
Now, if ex-Beatle output here is to include collaborative efforts, I’d have to think about exactly which song I’d pick, but for my money, the Travelling Wilburys were the best thing to rise out of the ashes of the Beatles. I think George Harrison was VERY talented, but most of his solo output was just so-so, just didn’t grab me, and as for Ringo, he should stick to the drums (if you saw “Walk Hard”, there was a great scene where the Beatles were all at each other’s throats and Ringo pipes up, “I’ve got a song about an octopus…” that summed it up pretty well for me on the Ringo front).