I liked your guide, so I figured I'd share some of my findings (I played around with nectar making way too much for my own good). Warning: This could be considered a spoiler.
Anyway, first the basics.
Each fruit has a value. To figure it out, it's as easy as making a "pure" nectar with that fruit and dividing the price of that bottle by 10. Here are the numbers I have for the fruits' values (assuming every fruit is perfect) - from highest to lowest:
Life Fruit: 40
Flame Fruit: 32.6
Cranerlet Nuala Grapes: 29.1
Gralladina Fran Grapes: 21.7
Pomelo: 21.7
Meloire Grapes: 18.1
Cherry: 16.3
Avornalino Grapes: 14.5
Plum: 12.7
Lime: 12.7
Watermelon: 12.7
Pomegranate: 10.9
Cherimola Blan Grapes: 9.1
Renoit Grapes: 9.0
Apple: 5.4
Grapes: 5.4
(For clarification, these are with level 10 nectar making, perfect fruits, all relevent gardening and nectar making challenges, as well as the "better quality" upgrade for the machine).
There doesn't seem to be any logic to what quality the finished product will have, though I wouldn't mind being proven wrong on this. What IS logical is that the quality acts as a multiplier. Its exact value isn't shown, but the "grade" is the range of the multiplier:
Amazing: 1.5 and up (highest I've seen is 1.68)
Great: 1.3 to 1.5
Pretty good: 1.1 to 1.3
Not very good: 0.9 to 1.1
Terrible: 0.9 or less (lowest I've seen is 0.63)
The rating makes a huge difference obviously.. Take 5 life fruit, 5 flame fruit.. That's a "base value" of 363, but the result is terrible. Final price: 251 (a 0.69 ratio). 4 life fruit and 6 flame fruit however taste great together, and are worth 503 for a base value of 355.6 (1.41 ratio). You get the idea.
From there it's fairly clear that a life fruit heavy recipe is probably worth the most. That is, unless there's something beyond "Amazing" quality. My candidate for "best in game" is this one:
9 Life Fruit, 1 Pomegranate
Amazing quality, with perfect ingredients and the right tools, that's §604 a bottle.
That's what I found out so far. However, there is a doubt: Either people on forums aren't very reliable, or there might be a variation on what comes out as "Amazing" for different people. Some of the recipes I've read about simply don't give the right results.. Regardless, here are some to play with.. All the "amazing" quality recipes I've found, sorted from highest "value ratio" to lowest.
5 Apples, 5 Renoit Grapes
--------------
Base value: 72
Actual value: 121
Ratio: 1.68
^^^^ Highest ratio, and a nice one to play around with early on. Both fruits are common and found within walking distance of the nectary in France. Doesn't sell for much though.
2 Gralladina Fran Grapes, 2 Cranerlet Nuala Grapes, 1 Flame Fruit, 1 Life Fruit, 1 Cherry, 1 Pomelo, 1 Avornalino Grapes, 1 Meloire Grapes
--------------
Base value: 244.8
Actual value: 408
Ratio: 1.67
6 Life Fruit, 3 Flame Fruit, 1 Cherimola Blan Grapes
--------------
Base value: 346.9
Actual value: 572
Ratio: 1.65
^^^^ Highest value nectar that gives both flame and life fruits buffs
2 Life Fruit, 2 Gralladina Fran Grapes, 2 Cranerlet Nuala Grapes, 2 Cherry, 2 Pomelo
--------------
Base value: 257.6
Actual value: 422
Ratio: 1.64
7 Life Fruit, 2 Flame Fruit, 1 Plum
--------------
Base value: 357.9
Actual value: 586
Ratio: 1.64
7 Life Fruit, 2 Flame Fruit, 1 Watermelon
--------------
Base value: 357.9
Actual value: 586
Ratio: 1.64
9 Life Fruit, 1 Pomegranate
--------------
Base value: 370.9
Actual value: 604
Ratio: 1.63
^^^^ Highest value nectar
5 Life Fruit, 5 Pomelo
--------------
Base value: 308.5
Actual value: 479
Ratio: 1.55
5 Life Fruit, 5 Cranerlet Nuala Grapes
--------------
Base value: 345.5
Actual value: 533
Ratio: 1.54
5 Gralladina Fran Grapes, 5 Apple
--------------
Base value: 135.5
Actual value: 209
Ratio: 1.54
I've also found 23 great recipes, but listing them all might take a bit too long, this post is already enough of a wall o' text
Hope that helps!