All of them
...just kidding!
As much as she's slagged off, I can't help but adore most of Geri's solo stuff, especially Look at Me & Mi Chico Latino.
Most of Mel C's stuff is great too (I Turn To You will always be a classic) and Victoria's stuff is so bad it's good. They've all done really good stuff featuring other people but idk if that counts as solo?