The Adrian bias is strong in this one.
Voted for Nicko because in my opinion Dave and H have contributed more to an essential Maiden guitar sound than Nicko has to a defining drum sound.
If not for Nicko, Maiden may have shifted to double bass, which would have drowned out the melodies and harmonies and not allowed the music to breath as it does. I think Nicko provides massive impact on each and every song. Especially POM, AMOLAD, TFF and BOS.This is a silly survivor. But anyway:
I love Nicko McBrain and I think he contributed a lot to the direction of the band. However, that was in 1983 and I think the band knows how to write for that style by now. IMO, the least important member can be defined by two things: 1) who contributes the least to the songwriting, and 2) who contributes the least to the overall sound of the band. Nicko has only written one song in his entire career with the band and, although I believe a strong drummer with personal style is highly important to a band, I think Steve drives the rhythm and that melody is more important in Maiden's music (and obviously has nothing to do with Nicko).