I like the Beatles, just not Lennon especially his solo career. I'm more of a Sir Paul/Wings fan. Band on the Run beats Imagine any day.
None of their solo stuff is anywhere near as good as their best Beatles stuff, in fact I think most of their solo albums range from terrible to only slightly above average. However, there is one Beatles' member solo album that is awesome, and maybe even as good as the best Beatles albums - George Harrison's album All Things Must Pass.
You want to talk about underrated? George harrison is by far the most underrated Beatle. In my opinion, George was actually the one that took the Beatles from average pop rock band and put them into the pantheon of rock and roll's all time greatests. Listening to their solos albums makes this quite clear. Paul's solo stuff is fun, but not too deep or special. John's solo stuff is always wild and musically simple in a very appealing way, but it's got no breadth, it's just all kinda the same and very often musically too simple. Ringo solo stuff is fine, but he also clearly wasn't a big part of the Beatles' tonality.
But then there's George. Listening to his solo album All Things Must Pass makes it clear that he was the one that added most of the magic. This album's emotions go from haunting, to funny, to mystical, to uplifting, and beyond. His album has the rich sonic tapestry that made the Beatles' so special. His lyrics are stimulating, his voice is emotionally powerful, his melodies catchy, his harmonies velvety, and his arrangements perfect.
If you're a Beatles' fan and you've never heard All Things Must Pass, then you need to go buy it. Tomorrow morning.