It definitely depends what you're looking for!
Since it sounds as though you are relatively new like myself, I can understand where you're coming from.
I have had the Mackie MR4's for about 6 months now. The MR 5's are quality as well. I'd also recommend the Rok-it 5" speakers, my good friend has them, so I have personal experience with all of these monitors.
I am a college student so I can assume that I'll be working out of a relatively small room for the next few years. A small room with thin walls at that. If you have a small room, I'd recommend keeping the monitors small. So, you'll probably want speakers with cones that are 4 or 5 inches at the most.
Since you'd be saving money with speakers anywhere from $150-$250, you should also get a small package of acoustic foam (about 12 sq ft should do). Place it on the wall behind the speaker, and most importantly on the side wall at the angle of first reflection. This will give you a much clearer picture of what is coming out of your monitors.
So thats my newbie advice! Hope this helps, and remember to read lots of reviews!
-Delta