They are all important. But they are all inter-related, so it is best to have a balanced system.

For example, if you have a lot of RAM, you will hold more in memory, and do less paging (to disk), so the HD will be in use less often, and so HD speed would be less important. If you skimp on RAM, and don't have enough, the HD will be used more, data will be fetched more slowly to the processor, so having a super fast processor may be wasted because you have a bottleneck in your memory/storage and the processor is idle waiting on data.

Similarly, if you have a good graphics card it may take some of the workload off the processor, making raw processor speed less important.

On a laptop, I think the main place you can gain performance improvement is in a good dedicated graphics card.

Next most important would probably be RAM. I have 8GB and doing heavy editing in CS6 that seems to be enough. 16GB might give some incremental improvement, but I expect 8GB is enough.

The HD speed will help files/thumbnails load more quickly, and speed up paging, but I think it's the least important, especially if you have sufficient RAM.

Processor speed I think depends on the software you use. Some make better use of multiple cores than others. The speed (GHz) isn't as important as how many cores, and whether you can take advantage of them.

There is an in-depth discussion of these on the Adobe site: How to tune Photoshop CS6 for peak performance