A lot of people consider singing to be the best possible means of self-expression out there. By communicating through a song, you can put forth a lot of emotions that people simply don't express over the course of regular speech, let alone in our daily lives. But then, when you make music, all bets are off and so are the proverbial gloves. You can express yourself incredibly through music, whether with complicated lyrics or the occasional scream and growl.
However, sometimes, your voice is simply not going to be enough to get the message across in musical form. Sometimes, using the right instrument can produce the sounds you simply can't do naturally. In the same vein, sometimes, some feelings must be expressed in a way that words can't do justice to.
Instruments to Pound
There's a strong, visceral quality that characterizes all of the percussion instruments. In music, it's rare to be able to beat on something other than a drum, and the effect can be felt in every cell in your body. When you need to express something that rages within, or that thunders off the walls of a canyon with raw elemental power, the percussion section is going to be your best bet for expressing it properly.
Granted, there's definitely some room for refinement in the percussion section. It's hard to hold a beat without percussion after all, and this is essential for virtually all music. But then again, where the drums really shine is in their ability to rattle walls and deliver hits that feel as mighty as a machine gun. Granted, sometimes, you need a more subtle approach than percussion really works for.
Softness and Warmth
Woodwind instruments let you express some very yin, soulful qualities that few instruments of different types can really do justice to. When a melody needs to express gentleness and happiness, woodwinds just can't be beaten. Even when you want to go a little maudlin, the woodwind section lets you express this set of feelings as purely as you could with the spoken or sung word. However, there are depths that the woodwinds simply can't quite reach.
Stringed instruments provide some of the most powerful depth and versatility of any of the instrument classes. With the ability to rail into happiness or scrape the bottom of the barrel of sadness, the instruments allow for some of the richest sounds of expression humanly possible. The sounds you can make with a cello or a violin are among the most variable in the world.
Also, you can customize your instruments as well as any others. By changing the strings, you change the tones. Even a beginning violin with a good set of strings can make some incredible sounds. Finding great beginning violins is also relatively easy, since the real power of this kind of instrument is lost on so many people. You can even slightly change the string tension for even more versatility.