This was tough until I really looked closely.
My first reaction was Diary because Diary is the best solo Ozzy song and might be better than anything he did with Sabbath.
But then Mob Rules has Falling off the Edge of the World, which is every bit as good.
For me, Turn up the Night is on the same level as Over the Mountain, and the same is true of Voodoo and Flying High Again. You Can't Kill Rock and Roll is a really good song, but Sign of the Southern Cross is definitely better.
This is where Mob Rules pulled away. Believer, Little Dolls and Tonight are all no better than average songs, but The Mob Rules, Country Girl and Slipping Away are all well above average, and the worst of them is better than any of the three songs on Diary.
Diary of a Madman and Falling Off cancel out, which leaves SATO and Over and Over. O&O is the only weak track on Mob Rules, and SATO is excellent.
With all that, I went with Mob Rules, but both albums are big contributors to the golden age of heavy rock/metal from about 1980-85.