OMG, I was going to ask this very question on Monday.
Here's what I found out.
Two types of systems: Wireless and wired.
Both are good but for "stubborn" dogs and sloping, irregularly shaped properties wired is the way to go.
Petsafe seems to be the most popular retail brand out there. They have a new stronger version of their wired system for hard to train dogs. Best price I've are here:
http://petstreetmall.com/advanced.aspx?id=3945&cat=481
Things to consider:
- progressive correction
- standard batteries if possible
- power low indicator
- waterproof collar
- sizing collar to dog weight
- long haired dogs need the longer probes
- if going wireless buy where there is no restocking fee if returned. The newer wireless systems state that they can be effective on larger/fairly stubborn dogs.
Well priced wireless with decent return policy:
http://mightypets.com/product.asp?3=277