Adrian or Bruce? That is a tough call. Those who would argue Adrian have a point. When he left the band the snap went out. They seemed to lack energy.
The last two Bruce albums didn't shine as much as their heyday, but can we really say it was Adrian's leaving that did it? Partially.
I think what happened to Maiden is what happens to every band when they become huge. Egos, complacency, divided entourages, etc., take over and the music suffers. Adrian leaving was a sign that the creative well had been drying up. The same with Bruce's exit. What caused this was not a lack of talent, but a lack of commitment.
Now that's out of the way, we need to argue both Adrian and Bruce's solo material. Adrian's ASAP is a decent rock album. His 2nd Psycho Motel album is really good, if not great. The 1st PM album is just ok.
Bruce's Tattoo is a fun rock record, but only Son of A Gun really rises to classic status. Then Bruce found Roy Z and I think we can say here is where the perfect union was forged. Balls, Accident and Chemical are all phenomenal records in their own right. But again is it Bruce or is it Roy Z or is it their combined chemistry? Probably #3.
Bruce and Adrian came back at the same time, so the success of BNW cannot be said to be one or the others success. Remember though, no they were older, some of the hunger returned and Maiden was back on track.
Evaluating the songwriting contributions of each within Maiden breaks out like this: Adrian wins the early period, but I think Bruce really developed as a songwriter after going solo and you can see this on the Maiden mach 3 releases. Adrian still does a good job, but Bruce's lyrics are deeper, his songs are rich and more multi-dimensional and because of this I pick Bruce.
Some of Di'Anno's tracks rocked too, though.