Well, this is a really subjective question, because different people use stuff in different way. So, this is only my take, other people may use these devices in another way.
Let's talk about a regular laptop:
It comes in different sizes and shapes and are generally bigger than netbooks and/or tablets. Laptops have generally more CPU/VGA power and a bigger hard drive.
They also are the most expensive (generally) between all of these devices.
Laptops, from my point of view, are useful when a person needs the power/capabilities of a desktop computer but with advantage of being portable. They are generally the top choice of persons like consultants, pc repair guys, or schools.
For example, an IT security consultant which goes to companies to test/resolve problems needs a laptop to move and do tests having the same software as a desktop PC.
Laptops do cost more, usually, than netbooks or tablets and are more heavy. Also, they have less battery life and that's why generally laptops are not really that portable in today's day, since you always see people move with the laptop + charger wherever they go.
Netbooks are "mini-laptops". They usually have less CPU/VGA power, smaller screen resolution, smaller hard drive. The good think about a netbook is that it offers the same operating system and the same apps as a desktop/laptop but in a size that is way similar to one of a tablet.
They of course cost less and also, generally, have better battery performance than a laptop.
I don't really love netbooks though. They are somewhat of a "hybrid" solution between a laptop and a tablet. You can't really run powerful stuff on a netbook and it isn't really as portable as a tablet. This solution is good if you need to do light PC-like stuff. But then again, if for you light is email, photo, web, social a tablet might be the right choice.
Tablets are the most portable/light object between the 3. A tablet is good when you need to read email, ebooks, google something, play simple games, watch a movie on the road, ecc. Of course, tablets are more restrictive than a laptop/netbook. You can't run all the windows apps you may need, you can't install linux if you want. Tablets are perfect if you want something simple to do simple things.
So, which one of these devices are more productive? As I said, it really depends on what you have to do. I personally choose the combination laptop/smartphone. If I want to get things done I need a good piece of gear and that's a laptop (when I am away from my desktop pc). The fact that it's heavy/needs charger doesn't bother me, since a netbook or even a tablet would need, in fact, something to carry them in. So, since you already need to carry something around, why not take it to the next level and get the most out of it, using a laptop? You can always use your smartphone to quickly record/grab notes if you need.