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).
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.
I think it's obvious that Janick is the least significant member, and then it would be Dave or Smith, probably Smith.
Which is kinda funny because Maiden is a guitar heavy band. It just seems that each guitarist is expendable where as Harris and Dickinson are unique and not so expendable.
Although, I think Halford, Kiske, Jorn, and Dio could have perhaps done a fantastic job with Maiden.
But, I want maiden to sound like maiden, not JP, Helloween/Unisonic, MasterPlan or Dio.
Harris makes Maiden sound like Maiden, but, in a way it seems that Harris has started to allow Maiden to explore new things, making AMOLD, TFF, BOS so great.