That is a difficult question. So much goes into what kind of sound you want and what type of guitar you are using and what equipment you already have.....the kind of room you are using and style you are playing also has a huge bearing. You have to try different mics as per the same reasons as vocals. A good workhorse is a Shure SM57 not to be confused with a SM58...or my fav an Audio Technica ATM4041 though anything from the 4000 range is good and designed for acoustic use.
Here is an article from SOS mag which gives some good advice.
http://www.soundonsound.com/sos/aug01/articles/recacgtr0801.asp