BLU has access to mage gear, so we can set that job aside for now since it can be categorized as a mage.
RUN has access to FC native merit JA, so it never has an issue there.
You're then left with - COR/RNG/NIN/MNK/DNC/THF. NIN has near-instant cast spells with the right setup, so no issue there.
WHO CARES about using a valuable reward ring for FAST CAST for COR RNG MNK DNC THF??? It's only useful when you're /NIN for shadows, which is niche to begin with - and barely an impact for the infrequent Utsu casting of any of those jobs even if you do /NIN.
Any one of those jobs have far better uses of a reward ring than a FC piece.
DNC THF: Karieyh for Rudra's
COR: Haverton (best Racc ring, only snapshot ring, DW option) or Karieyh (best Savage Blade ring)
RNG: Can use Haverton for the same reasons as COR. Weatherspoon is indeed a great RNG choice, but that's due to being by far the best TF ring... it's sure as hell not for /NIN FC when you rarely need to /NIN (/DNC or /WAR are nearly always better in any situation but super rare cases of some deadly long-range blinkable AoE requiring shadows - I've chosen /NIN over /DNC like twice in the past year on my Ranger).
MNK: eh... The real answer is that none of the rings are that amazing for MNK, so just don't make your selection based on MNK. But if you had to pick MNK gets even less out of Utsu FC than the rest of the light DDs (since it goes /NIN less often than any of those jobs). I guess maybe Karieyh for Howling Fist, or perhaps Vocane.
You also left off
PUP, which does suffer from a lack of great FC gear even relative to the other light DD jobs (no Carmine, no Adhemar, no Ambuscade set pieces with FC, etc.) But again, there are other better ring choices (Thurandaut) if you're a die-hard for that job.