This is entirely dependant on what parts they play in the project.
I think Adrian's would be the most outstanding musically and would have good vocals to boot even if he did it himself.
Steve... can write great songs, but I don't know whether he could play them at all other than bass, and his vocals are not exactly great.
Janick would have some interesting compositions but be very hit and miss I imagine from track to track.. vocally, no idea!
Dave would be very melodic but not overly outstanding, he doesn't do a massive amount of writing in maiden compared to the 3 main writers, and I think it works as it means when he does do something it's fantastic! trying to stretch it out may hurt it.
Nicko... I'd pay to have an album sung by nicko without a second thought! But if he wrote the songs it would probably be pretty subpar I'm sad to say.
As discussed on chat, IMO Maiden should release Nicko singing Rime of the Ancient Mariner, and then when next years 'facebook anti-xfactor' campaign rolls around (I disagree with the principle as it's just handing it to someone else the same way that xfactor does) we should push for that to be the song they campaign for!!! Nicko/Rime as christmas number 1 would be fcking immense.
Anyway, I voted Adrian.