positive

this + NOUN PHRASE + of + POSSESSIVE PRONOUN

C2 highlight something, often in a positive way with demonstrative DETERMINERS:
‘this’ + noun + ‘of’ + possessive pronoun
(English Grammar Profile)

In the English Grammar Profile, B2 point 124 in the category of modality is defined as using:

the fixed expression ‘I must say’ to give emphasis, usually about something positive.