In my opinion, if you're willing to spend the time, wet blending (two paints, mix them to a smooth gradient while both are wet) offers the best, most reliable results. It's really good for areas like capes, and it's often the only solution if the space is small, like a raised jewel.
Glazing (using a medium like Lahmian that allows you to have a little more on one side and a little less on the other) is faster, and works really well on the types of surfaces you might use a wash on (ie the model is not just flat), and works well on things like power swords, where you have limited space. You can also glaze white, which is cool.
Feathring (using water to dilute and soften the edge of one paintonly -- the other color is dry) is great well when you have more space to work with, like the top ridges of an Imperial Knight. It lets you have a soft transition quite easily between two similar colors. The
PP way of doing it is called "two brush blending", I think.
The three easiest ways, which aren't really blending, are layering (which creates bands of color), drybrush + glaze, and airbrush. You can make the layering quite convincing if you're willing to do a couple extra layers. The trick is to have a consistent amount of space between each layer. You can also just hit it with a drybrush, glop some glaze it, and it will look decentish. Airbrushes are pretty self-explanatory as to why they are good
In short... there are just lots of ways to get the job done, and I think depending on what you're comfortable with and the space you have to work with, all of the techniques have merit