Nowadays external monitors vary in size, specs, and options. You didn't say if you wanted a high resolution monitor, one that's IPS, one that has USB ports, speakers, etc.
The best monitors in my opinion other than made by Apple, are from BenQ and Dell. You can go to both of their web sites and see them for yourself and check out the prices. Also, HP just came out with a very nice high resolution monitor that's IPS. Check them out too.