In a way, that's a loaded question. It's like asking which running shoes are the best, or which automobile is the best. Of course, it depends greatly on who's buying.
Fortunately for us, Garmin has a very extensive selection of products with a very broad price range. I especially am fond of their newer nuvi line. You can get a Garmin nuvi anywhere from $150 to $900. Of course, there are more and more cool features as you go up in price.
If you're just getting into GPS devices and you want an entry level unit, you can do no better than the Garmin nuvi 200. Priced at under $150, the nuvi 200 does everything that a basic GPS device needs to do. It has a touch screen interface, spoken directions, pre-loaded maps, and a large database of Points of Interest (POI's). Of course, it's backed by Garmin's commitment to building quality equipment.
Before you pull out the old credit card and buy the nuvi 200, however, you might want to consider spending a little more money.
For only about $100 more, you can get a Garmin nuvi that actually says the street names, instead of just telling you where to turn, is Bluetooth compatible, and has maps of all 50 states plus Canada. The nuvi 360 does all of this and more for slightly under $250. I love Garmin's quality, and for a mid-range product, I can not think of a better buy than the nuvi 360.
All the 360 lacks is a larger screen, multiple point routing, and maybe maps of Europe. To get that, you'll have to go up into the $300's and snag one of the nuvi 600's. The 660 does everything the 360 does, but it has the larger 4.3-inch screen.
Is the larger screen necessary? Personally, I like it, but then I've got a fairly large car with a large windshield. The 3.5-inch screen is certainly completely visible in any car. It's not like you need the larger screen so that you can actually see the device.
If you want to go all out and buy one of the top of the line Garmin GPS units, then consider the 880. The 880 can do anything any other GPS unit on the market can do, and probably do it better. It costs $900, but then it was only a few years ago, when nearly all GPS units cost that much.
If you buy the 880, you know you will not want to trade in any time soon. So, in a sense you might be saving yourself some money. It's kind of like buying an expensive pair of shoes you plan to keep for several years, as opposed to buying a cheap pair that are going to wear out by Christmas.
Bottom line: it all depends on your needs and your budget. If you buy one of these Garmin GPS units I've mentioned, then you can't go wrong. Actually, you can't go wrong buying any Garmin product. They're a great company and they make a quality product.