Choosing the "best" mic depends on how and where it will be used, but there are some general considerations that can help you narrow your search. Condenser mics are generally higher output than dynamic mics, so feedback can be a problem depending on the room and the polar (pick-up) pattern of the mic. For live use in multiple locations, you will probably be best advised to look at mics with a narrow pickup (polar) pattern because they offer the most off-axis rejection and thus have the lowest feedback potential.
Potential feedback increases as the polar pattern widens, so feedback rejection from greatest to lowest is as follows: shot gun > super cardioid > hyper cardioid > cardioid > figure 8 > omni. A shot-gun pattern is very narrow and an omni is very wide. A shot-gun mic is usually too narrow for consistent results (any movement of your body will impact the volume as you move in and out of the pickup zone), so super cardioid and hyper cardioid patterns seem to offer the best combination of feedback resistance and practical use.
The next consideration is tone. Just as all guitars don't sound the same, mics will impart some amount of sound coloring...some will impart more than others. So, it is important to test mics with your voice and guitar to find the ones that most accurately capture your sound, or at least will flatter or improve your sound. Yes, some will actually make you sound worse!!! Of course, as posted by m-thirty-great, EQ'ing can compensate for some of the mic's characteristics, but if you can get the best for your sound to start with you will be way ahead!!
Another consideration will be reliability. Some condenser mics are extremely sensitive and fragile, not all, but some can be. Typically, dynamic mics are more rugged than condensers, so it is no surprise that they are used more in field and live applications than their condenser counterparts. However, many condenser mics are built well and have proven to be robust enough for the rigors of live performance application. Do your research!!!
I hope this helps...I'm sure someone will offer a differing opinion, but the above should at least give you some direction.
