An LCD monitor or CRT?
For CRT, this is good:
19" or bigger
.24mm dot pitch (if it's 19")
Flat picture tube
Good refresh rates, usually 85Hz @ 1280x1024, 75Hz @ 1600x1200. The higher the refresh rate, the better.
For LCD:
Good contrast ratio, 500:1 or more.
Fast response time, 8ms or less.
No dead pixels
17" or bigger
DVI-D support