Probably the Magellen Explorist GC. It's got the caches pre-loaded and is designed especially for geocaching. Should be able to pick one up online for under $150. I don't own one but I was at an event with a guy who had one and it seemed to keep up with the ones everybody else had. He said that he loved it.
You won't go wrong with a Garmin eTrex Venture HC. There are cachers that own one of these and have found 1000s of caches with them.
Personally, I want a unit that I can download pocket queries into or at least the names and coordinates of a cache. OK, I've got a Delorme PN-20 which is a fine unit, however it's a real pain in the butt to try to hand load coordinates into it. The point of this paragraph is that its worth spending more money for something that you can get the coordinates and other information into easily. A unit with paperless geocaching capabilities is worth it also.
If you want to go rock bottom in price you can get a Geomate Jr.. But you have to get the update kit with it. The update kit consists of a connector cord so that you can plug the thing into a computer to download new coordinates. I own a Geomate Jr. I got lucky and got it at Target on close out for about have the price. You cannot enter coordinates into a Geomate Jr manually they have to be entered off of the computer. So a Geomate Jr will cost about $70 and the update kit will cost you $25 so unless you find a bargain you will be into it for $100.
If you get anything from the Garmin Etrex series make sure that its got an H in the name. That stands for "high sensitivity" Further, I recommend that you get a newer model that has a USB rather than a serial port like some of the old Garmins did.
You should be able to get a Garmin Etrex H for about $70 on ebay or amazon. It will do you fine to get you started. But the additional features of the Garmin Etrex Venture HC will be well worth the extra cash.
The Lowrance brand offers a lot of features for a low price but I do not recommend them. I haven't seen any geocachers use them. I asked about them and have been told that they are of poor quality and unreliable.
