My search skills fail yet again. So, bear with me here, we need to consider Ringo as his first solo album for this to have any chance of working; since he called his 6th album Ringo the 4th, it seems he thinks so too, and we may not be stretching too far. So, their first solo albums after Let It Be was released:
John - Plastic Ono Band
George - All Things Must Pass
Ringo - Ringo
Paul - Ram (this one is likely to be the most contentious, I know lots of people like other albums of his more)
But I think that first ...