ne plus ultra
Concise Oxford English Dictionary © 2008 Oxford University Press:
ne plus ultra /ˌneɪ plʊs ˈʊltrɑː/
▶noun (the ne plus ultra) the perfect or most extreme example.
– origin L., lit. ‘not further beyond’, the supposed inscription on the Pillars of Hercules prohibiting passage by ships.
'ne plus ultra' also found in these Oxford entries:

