If we talk about plants that are the same quality (let's say Perfect), and not weird special ones that need to be unlocked, which one is the most efficient? It might not necessarily be the one that is worth the most, if the time between harvests is different for different plants. Which plant has the best money/time ratio?
Farming is, just like pretty much everything in the game, fairly overpowered in terms of money production; but even so:
While I haven't done the math, I believe the most efficient plant to be Life Fruit, with each piece of perfect produce being worth a whopping 100 simoleons (more if sold at the grocery store, and even more if sold at the consignment store).
However, for some real money gain from farming, grow Life Fruit and pomegranates:
The single most valuable wine known to simkind is 9 Life Fruit + 1 Pomegranate; with a few plants you'll be able to pull down thousands of simoleons per day, even with mediocre quality ingredients.
Other efficient plants are limes and garlic, although the garlic is perhaps best used fertiliser for your lime trees.