Walls and Bridges is a good, underrated solo Lennon album. "Surprise Surprise (Sweet Bird of Paradox)" from that and "Out the Blue" from Mind Games might be my favorite solo Lennon songs. Big fan of "Watching the Wheels", here, too.
Overall, though, I think pre-80s slump McCartney had more strong albums than the rest of them in their solo careers. Ringo's first album is great. I'd put All Things Must Pass at #2 for their solo albums, behind RAM, and after that, Harrison actually has a lot of great songs that almost never get played (I can list some favorites, if anybody cares), but like Lennon, not all his albums hold up all the way through.
I'd say Lennon has the critical view on his side over McCartney because people expected something "important" from the most famous musicians in the world, and Lennon delivered them angry, painful, confrontational music out of the gate. As people go back and actually listen to what McCartney was putting out in the '70s, without the burden of expectations, I think his reputation will improve, especially for the lo-fi ones that have been praised in this thread, many of which could probably be put out today and get a lot of praise.