What would be the most profitable crop that I could raise in a greenhouse using hydroponics? The most common answer has been tomatoes.
I don't have a lot of experience with commercial greenhouses, but have also heard that tomatoes are you highest yield in incomes per unit floor. I think this is because they vine, and more volume of greenhouse can be use in the same time periods. That's the key into greenhouses = energy (light and heat) in to the yield of marketable products outs.
New York City has found Basil to be very profitable during the winter months growing fresh Basil for the Italian seasonings markets. Pesto and many other dish preparations require fresh Basil off-season. So the Finger Lake of upstate New York now grows fresh Basil for the winter market needs of NYC.
I have also heard that growing most spices in regions where there is little farming and/or transportation costs become massive also allows green housing to be profitable. Winnipeg is such an example, where they grow most of the culinary spices for the restaurant trade, both winter and summer.
So, selection of greenhouse products should be reflected in local market demands, not those for export. This requires some due diligence, as a greenhouse cannot compete with field production crops. This is especially true for dehydration and further processing. Greenhouse are always a better solution toward some fresh products, especially during the winter when imports are so expensive.
Unfortunately, this is becoming less and less possible with FedEx and the new overnight delivery services so popular now. One California farm (3,000 acres) now provides off-season crops for the gourmet restaurant trade for almost half the country now.
There are a number of tabloids and magazines devoted specifically to these kinds of crops, but I am mostly a field man. Sorry I can help much further, with limited background and different forms of farming.