One of the key techniques I use in my instrumentation is Physical Modeling. This type of synthesis has a unique and organic sound. Wikipedia defines it as:
In sound synthesis, physical modelling synthesis refers to methods in which the waveform of the sound to be generated is computed by using a mathematicalmodel, being a set of equations and algorithms to simulate a physical source of sound, usually a musical instrument.
I also really like this image as a reference to how Physical Modeling does it. You can see how they have broken an instrument into its parts and used different synthesis to achieve a sound that emulates it.
Here is a video on my favorite Physical Modeling synth and an overview.