If there's a workaday mic in the world, used by every studio in existence as well as on just about every stage, it's the SM57.  Generally regarded as about the best general-purpose amp mic there is, it's also used for instruments, vocals (yes, vocals), ambience, drums, and anything else you can think of.  No mic fits every circumstance, voice, or instrument, but I guarantee you, the SM57 is right in the top dozen or so on the list when it comes to trying to fit a mic to a given situation.  It's indestructible, sounds good, and is inexpensive.  You really don't have a complete mic kit unless you have a few.  I have several, and use them all the time.  Look for deals and buy them when they go on special.