Just buying seed based on the time it will take them to grow is not the best strategy. Most importantly you must know how often you can log into your Facebook account and access Farmville. It will make no sense if you plant a crop that will take four hours to grow and you can't get back to your computer for ten hours or more to log in and harvest them. The plants will have withered and died and will be worth nothing. You must plan your crops according to when you will be able to attend to them. If you know you will be away from the computer for a long time, plant things that will take longer to mature, 1, 2, or even 3 days maybe look at having trees or animals.
These do not require as much attention as crops do, and can still make money if neglected for a long period of time. With trees the fruit will fall off and be lost but the tree doesn't die and you can still make money when the next batch of fruit ripens.
After level 20 when your farm starts to spread out you will have to start buying some machinery.
I remember my own farm up to this point I had 100 plots - 10x10 and it took a long time clicking 100 ploughs, 100 sows, and 100 harvests day after day. So consider using tractors and seeders to get your planting done. With a 25 percent time-saving, this can make a huge difference on a big farm. But you must weigh up the expense of harvesters and planters and what your own time is worth.
All the seeds you plant and the omnipresent XP that you are building up have a strange and intertwined relationship. The ratios have very little bearing on each other, and are specifically made to be treated as 'trade-offs'. You need to build these trade-offs into your Farmville seeds strategy. There is no such thing as the best seeds to plant. Earnings change as you level up and get access to all the items that have been locked away from you. The seeds you plant will either earn you a lot of XP and little cash, or the other way around lots of cash while bringing in less XP. Some crops are very expensive to buy and will get you very high XP but usually take a long time to grow.
After level 20 you seem to gain some sixth sense as to what seeds will be the most beneficial in terms of either coins earned per hour, or XP earned per hour. You can sort the seeds into the best earners for your present level. Make sure you adjust your list to keep it current, since things change as you level up. It is best to compile your own lists, since the game does get updated and the prices and values of seeds and crops can change from time to time.
