Well, turnip will be your best friend in Spring, since it's the first Spring. On Summer, go wild with pineapples. Sweet potatoes for Fall, and nothing for Winter. --- "Eirika, Level 20, 7 strength. Should I be concerned?" -Czardia "It doesn't take that much Str to make a sammich, so she's good." -PokeAMon
Cabbages in Spring which takes around 15 days. Pineapple for Summer... In Fall, plant a lot of sweet potatoes. Don't forget to hire Harvest Sprites if you want to earn a lot of money quickly... --- "That's not what I had in mind when I said I needed more RAM for my computer!'
If you're still training your sprites, go with Strawberries in spring. The difference in profit is fairly small, but your sprites get a TON more shipping experience to help them out later when you get sweet potatoes.
Your first spring you should focus on shipping 100 of the 3 store crops (Turnip, Potato, and Cucumber) so you can unlock strawberries for the second spring so you can use those for lots of sprite shipping experience for the second and every after fall, where you can make over 60k a day from sweet potatoes.
If you're not too far in and don't mind starting over, the year one spring FAQ is a pretty good guide. There's a few things wrong/you can do better, but for the most part it's not bad.
The big thing is to get started with the sprites asap and abuse them for their sweet, sweet slave labor.