Here's a list of top ammunition reloading die manufacturers in the USA. Each of the companies listed would (of course) like you to believe that they have the best dies ever made, and the truth is that each one has its benefits and shortcomings. Some are better than others, but for the most part each has a unique set of features, quality, and service.
Lee Precision has made a name for itself by making quality loading tools at reasonable prices. Comparing the cost of Lee dies with those of others will leave you wondering if Lee dies are any good, since they are priced so reasonably. The answer is yes - they are indeed good dies. Their Factory Crimp die is one of my all-time favorite reloading innovations, and even if you like to use another brand of dies, you can benefit from a Factory Crimp die - especially when loading for tubular magazines.
RCBS is a favorite of many loaders, and there's a reason for it - they make good stuff, and they stand behind it. Cost is more reasonable than some other manufacturers', and all of their reloading gear is guaranteed forever. Most of it is built to last a lifetime, but even if it doesn't, I and many others have found that RCBS takes their guarantee seriously, and they happily replace any defective - or even worn-out - parts.
Hornady also makes quality loading dies. Whlei I have never used any Hornady dies myself, I use their bullets almost exclusively for loading my big-game deer hunting ammo. If their dies are as good and as cost-effective as their bullets, then they are definitely of good quality.
Redding is known for high quality and close tolerances. One of the most innovative die-makers, it's certainly not one of the lowest-priced... but their products continue to be in demand by folks looking for the very best they can find.
LIke Redding, Forster has a good reputation for making high-quality, precision-made loading dies and other equipment. In fact, as of january 2004, their Web site boasted that their products were the choice of the United States Shooting Team.
Lyman has got to be the oldest maker of reloading tools in the world. For many years they were THE source of loading tools, and their products were often included with cartridge guns made in the latter part of the 1800s and into the 1900s. Nowadays, Lyman faces stiff competition, and unfortunately has not met the challenge as well as they might. I'm not fond of their dies, compared to those from other manufacturers. Truth be told, I usually look right past Lyman when I'm shopping for dies.
Dillon Precision is a fairly unique company... they make some proprietary stuff, as well as dies that work in industry-standard presses. From what I gather, they make good gear - and charge very high prices for it. Customer service, in my experience, has been good but not great. I own a Dillon Square Deal B loader, but I won't buy more die sets for it. They're only available from Dillon, and cost about $80. That's just too much for me. (Their other presses will accept other manufacturers' dies.)