Robin,
György Ligeti (and Olivier Messiaen, for that matter) would have certainly been on my list had they not done us the tremendous disservice of dying.
I agree with you that judgements of this sort are usually made with 20/20 hindsight, but certain composers have an undeniable buzz around them today. Our crystal ball gazing is to identify which ones the neu Mendelssohn might "re-discover" and why...(?)
Best,
EC
PS: for those of you unfamiliar with the Polish composer Krzysztof Penderecki, please go here:
http://en.wikipedia.org/wiki/Krzysztof_Penderecki