I'm a great George fan, I really love his work being a Beatle and most of his solo-stuff too. But in fact: he was not a brilliant singer in a way guys like Ray Charles, John, Paul or someone like the great Aretha Franklin was. But it's also wrong saying he was a bad singer, he was NOT!!!!!
Listen to the Beatles harmony vocals, he was very important for the classical Beatles-vocal-sound. Best example is the acapella version of Because (Anthalogy 3), in Germany one say's "I'm getting goose-pickels listening to that", I don't know the english equivalent to that, but I'm shure you will know
What I mean: he was a excellent harmony singer, he also proved that later with "The Traveling Wilburies" !!
I think the quality of his solo singing depends on the songs:
"Something" for example features great singing of George, strong feelings!!!!!!
Other songs remain kind of pale like "You like me too much" on Help and others.
Anyway, I love George, he's may favorite Beatle, although John and Paul always were the better singers, but in fact : I love them all!
The Beatles are the biggest band ever!