which dishes gain the most money?

I think the best way to find out which of the dishes gain the most money is to calculate how many profit/costumer you'll gain.

You have to subtract the costs from the total earnings and divide it with the servings. Example:
Mystery meat costs $8 to prepare it and you earn $12 , so the profit is 4$. Then you divide it with the number of servings, so you have to calculate $4/12 servings. Now you know, that the profit you gain for every costumer is $0,333.

('total earnings'-'costs') / 'servings'

Important is that there is always eating to serve, otherwise you can't get money.
My cafe is level 11 now, and I always cook "FANCY GELLO MOLD", because at the moment it has the most profit per costumer ($1,8625/costumer).

Additional I would recommend to cook at the night, because your zombies will not consume as much energy.

I have a different theory. I believe that it is not profit per serve but profit per hour spent preparing that is important. My reasoning using the example above of Gello Mold, just basic.

Using a similar formula:
(total earnings - costs)/hour

Gello Mold:
1200 - 475 = 725
725 / 12 = 60.41 per hour of cooking

Caramel Adams Apple:
780 - 27 5 = 505
505 / 1 = 505 per hour of cooking

Therefore in the 12 hours it would take to cook Gello Mold a total earning of $6060 could be made by cooking Caramel Adams Apple.

The other benefit is that in the 12 hours taken to cook 400 dishes of Gello Mold 3120 dishes of Caramel Adams Apple will be made allowing for a longer serving time as well.