..no mentions of Ken anyone? As a bass player, he would top my list..
I never saw Ray Williams play live, and never saw Will Youatt play with Man, only with Alkatraz, so cannot really comment
"Dancing" Ken Whaley was a great addition to what was a fairly static band, albeit that Man were not a "dance" band, he got things moving (possibly the opposite to John "the Ox" Entwistle)
John McKenzie was the ultimate musician - albeit possibly better as a session musician than on stage - look at his track record, Eurythmics, Bob Dylan, The Pretenders, Peter Green, Steve Hillage, Dexys Midnight Runners, Alison Moyet ...... and introduced different rhythms to Man's music
Martin Ace (me and my friend Duncan used to call him "one-note-bass Ace") provided a solid base (or should that be bass?) for Jones and Leonard to wander off from, and, (ignoring your pet hates be they Walrus, or Popemobile) was a good songwriter (e.g. MACBFGU)
So, if I'm forced to vote, it's either John Whaley, or Ken McKenzie