All depends on what your needs really are. I would recommend one that is completely waterproof and carries some sort of warranty. I'm sure you'll geta good bit of feedback from some peoplethat own one. From what I remember, the SD-1200 or 1800 were supposed to be quite a bit better than the 400 and 800 series, but obviously cost more. Go with the best you can afford.
I had an old Innotek that got ripped by alot of people but it worked for me and my lab at the time. When I got my pointer, I looked into one with more variable settings and more range. Mine also has the ability to set just on vibrate, or with stimulation & vibrate, or just stimulation. It's a DT Systems model and I can't say enough good things about it.

http://www.dtsystems.com/products/st100.aspx

I havean older version of the single dog ST-200.

Good luck.