It depends on your needs to a large degree, and it is a...

It depends on your needs to a large degree, and it is a confusing field.
Altium is the most expensive PCB design tool by a long shot, as such it's well known through corporate sectors, if you work for a large company, chances are you will be using Altium for PCB design, everybody knows it in those circles, and the company can afford the money. They were the first with a few features, but a few others have caught up so there's definitely competition, but Altium is still the favourite in the corporate world and if they have the budget, they have Altium. The Altium/ Solidworks partnership will be lucrative for 3D design as well.
If you're on the lower end of the market looking for something cheap, the Altium is not the best, however they have developed free options to capture that segment of the market and upcoming engineers learn the software
Main competitors at the moment include; Eagle, great range of features, but can be frustrating to use at times, it's simply not intuitive, they need to work on that, I think they are the second largest competitor, they are better for small businesses based purely on price point. Cadence OrCad, again, awesome software, known to be a little bit more difficult to use but not insurmountable by any means, but perhaps that is just subjective, it has the full range of features and strong market share as well, it would be my second choice, so like Altium a design created shouldn't be an issue to find components for with these two products, Altium having purchased Octopart is going to have an edge in this field soon though, being able to source any part they want, or knowing which parts are able to be sourced is going to make it a better product, it's not a crucial thing, but the little things do count.
Diptrace is after that IMO, it's little bit of a lower end priced product and it's not often going to be present in corporate gigs, it's got plenty of features, there's not really much anything missing, you can purchase "levels" which change number of pins, signal layers you can have ect, so basically as you get more complex you have to upgrade, clever little program, reasonably priced.