I was going to post earlier but I didn't.
Why?
Because the question can not be answered (said Leonard Nemoy spock style).
there is no "one" organic ingredient that is "best", even at flowering.
The best i could come up with is compost, but good compost is a mix of many things. In my case coffee grounds, leaves, chopped-up fruit peels, a little diluted and carefully handled pee.
Even then i'd hate to be left without my dolomite lime, bat guano, and sea weed extract for transplanting.
Ok...Because you been so nice NCG...
For one gallon of soil topdress or in "quick mix tea" one gallon:
1 increasing to 2 tbls. Fox Farm high P bat guano (has soil myccorrhizae too!)
1 increasing to 2 tbls. Boxed organic fertilizer without blood meal (Dr. earth or sweet earth)
pinch of dolomite lime to taste (of the plant) I use about 1 half tea spoon more if I add,
2 tbls much or "rinsed" bag mulch.
Shake this stuff up in a medicine bottle dump on soil surface water in, I use RO, my tap water sucks.
Or, put into gallon jug shake, shake, shake some more. Pour.
Sorry that is as simple as I can make it. I did buy a air pump the other day. i can now make my own teas. In a way this is an answer to your question, the teas rock. I bubble my tap water (go figure) for about 24 hours then put some EWC, kelp meal, boxed ferts, molasses, mulch, dolomite lime, and oatmeal-pan-rinse-water. bubble for 24 more (I'll take that, thanks bill) and feed to my plants. i'm not "Growing" right now but my Morning Glories and runner beans PUMP after I give them a dose of tea. Just siphon out of the 5-gallon.
Pure Blend Pro (Oh, Gosh I said it), not truly organc, i have a bottle and if i ever get three jobs and two girlfriends (too busy to be a purist organic gardener) I will use it again
The riddle answer: Knowledge
The bar no expense answer: money for the Good bottle of stuff
peace out