modality N1 rare writtenliterary
この上ない — the utmost
この上ない ・ このうえない
A full written guide for この上ない is on the way.
This point is mapped in the grammar tree, but its examples, formation table and usage notes haven't been written yet. The fastest way to learn it now is to meet it in real sentences inside Jengo.
See この上ない in real sentences
Jengo shows この上ない the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.
Study it in JengoSources Compiled with reference to A Dictionary of Advanced Japanese Grammar.