As much as I am embarrassed to admit it, I have been sucked into the attention black hole that is FarmVille. I blame Tad. He drug me kicking and screaming in and now I can no longer escape it's pull.
So having surrendered to it, the tiny bit of geek cred left to me wanted to redeem itself. I realized that the information on the crops was not really formatted in a way to help me make a good decisions about which ones to plant to gain the most coins and most XP efficiently to facilitate leveling up and buying more enhancements quickly.
Therefore, I put all the available info I have at my current level (14) into a number crunching Google Docs spreadsheet to produce these lovely informative graphs. I figure a bunch of you other addicts out there might appreciate them as well.
You can find the live graphs here. I will add to them as I level up and have access to more crops. Below are captures of the graphs at post time:

Some interesting conclusions:
Raspberries are great if you want quick cash (and will be there every 2 hours to harvest), but won't help you level at all.
Aloe Vera is a good all around choice being a decent rate for both Profit and XP per hour. And it only takes 6 hours to grow so relatively quick turnaround.
At time of post, White Grapes (a limited availability item) are a good second to Aloe Vera for all around leveling. When they're no longer available Morning Glory will be the runner up.