Besides cilantro and scallion, basil would be my next favorite herb. I like greens, so why not 'grassy' greens? Herbs have ben lauded for adding flavors without the calories, though let's not forget the health benefits as well.
I like cilantro because it pairs really well with other vegetable dishes and also a clear broth soup. It has anti-inflammatory benefits, lowers blood sugar, and helps with digestion. Cilantro is commonly used in Mexico and Asian dishes.
As for basil, the first cuisine you'd think of is of course - Italian. Even the color green is part of the country's flag. But basil is actually a native of India, which has a different name - tulsi. Basil also has its anti-inflammatory benefits, plus a cure for motion-sickness and treatment of respiratory system disorder. I like to throw in some basil right before I serve a dish of pasta or vegetable dish or shred some over a mixed green salad.
Of course, there are other herbs out there - fennel, thyme, sage...but when it comes to versatility, I put my vote for cliantro and basil.