Guitar: The guitar can be made to sound like anything, and yet its own sound cannot be duplicated accurately. They work in pretty much any musical context, from the heaviest metal to the catchiest pop to the smoothest R 'n' B. They even work in music some people wouldn't even call music. Plus, put a guitar in nearly anyone's hands and watch their sexiness grow (though in some cases, it'll drop as soon as they play a note).
Bass: Say what you will about bass players, but without these, you just don't have a groove. The bass can either be the bed on which the other instruments lay or step out in front to take the lead itself.
Synthesizer: Music sounded good before these were invented, but it's never sounded 100% the same since. I imagine the reaction of musicians (and listeners) to hearing electronic music for the first time to be akin to painters discovering a new, never seen before color.
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