There's definitely arguments for them all.. It depends on which you're best at / which you enjoy the most.
If I had to pick one, I guess 'Application Development' would be most 'essential' to have in the future. If you are looking at a career, then things (projects / applications / interfaces / technologies etc) are always being developed - its a natural progression in technology. Therefore, logically, you would probably be able to have a lot of work, or lots of projects to get involved in - and keep yourself in a job.
However, that's just logically thinking. If you are super hot at something like networks or infrastructure support - if you enjoy the area (it's usually no coincidence that you're good at it and have a good understanding of it too if you enjoy it) then you will be in demand / able to find work.
Some of the areas like storage management and systems management are often already set up and based upon set algorithms and rules. Therefore, logically, you would be entering an environment that you wouldn't necessarily have an edge over many people, who are already in the field (in fact they would probably have the experience already).
Not saying don't try these areas - if everyone took that view there would be no one doing it! Also, people often move around / migrate in I.T, and there are a hell of a lot of existing systems that need maintaining and managing correctly that already exist - not mentioning the new ones that would be created...
There's a lot of luck / hard work involved. Most of the time it's VERY tough to find a job that you have studied for, you need to be flexible.
