Having pondered long and hard as to what might be the pottiest alphabetic spelling in any language, I have finally come to a decision. I considered:
ss → [z] in English scissors, possession etc.
Another candidate was Portuguese êm → [əiəi] (approximately, but nasalised), as in têm “they have”.
However, I award the Golden Doozy Prize to Donegal Irish for
-adh → in words like bunadh “origin, people, kindred” and many, others.
In Connacht and Munster the sequence is simply [ə], but where on earth did come from?!?!
əð > əɣ > əʊ > uː
Figures. Thank you, Michael.