This depends largely on how intense you want to get. Do you want a full, number crunched answer? There are lots of tools out there that you can use to calculate your profits based on starting resources. This one is pretty popular, as it shows results in a graph and allows you to specify how much space and money you're willing to invest. There's also the one I use, which shows what crops to plant on what day in order to get the most out of your current resources.
However, if you're just looking for a simple, no-thinking-required answer, the best crops to get per season are as follows (based purely on a profit/day or profit/month value, which is not always the best measure of investment):
Spring: Strawberries provide the best profit per month if you can plant it at the start of the month, but Rhubarb (purchased from the Oasis in the desert) and Cauliflower are the best profit per day.
Summer: Blueberries are absolutely the best crop to plant, having the best profit per month and per day. Starfruit is second best, with Red Cabbage trailing not too far behind.
Fall: Rare Seeds (bought from the Traveling merchant) provide a massive profit (both per month and per day), but chances are you don't have many of them since they are hard to come by - even harder than ancient fruit. Cranberries are also extremely good on a per month basis. In third place is Yams or Artichokes.
Winter: You can't grow anything in winter other than the winter forage seeds, so the choice is pretty easy here, don't you think? :)
In my opinion, if you have enough seeds and a greenhouse, the absolute best profit maker is the ancient fruit seeds. They take a while to collect, but with a seed maker you can quickly fill your entire greenhouse. Combine with a large keg setup (in your shed preferably, though anywhere works), and you can easily make a huge fortune, though it might take a little time.