The Field Notes brand has become my favorite. It's simple, thin, and can be carried in the back pocket easily. I had been carrying the original version that is pictured above with the cardboard type cover. It however doesn't hold up well to hard use. I mean most notepads don't anyway.
In my former career as a Marine, I used Rite in the Rain notepads. They were waterproof and held up quite well. However aren't as compact as the Field Notes; and have a spiral metal thingy holding it together that I didn't like.
As if Field Notes couldn't get any better. They recently introduced their waterproof/indestructible version known as the Expedition Edition. This version has a blaze orange front cover and black rear cover. The blaze orange helps to maintain accountability of your notes in the field. In case they are dropped on the forest floor. There is also a subtle varnish imprint of a topographic map of Antarctica on the front and back cover.
The pages are filled with Yupo Synthetic Paper that is extremely water/tear resistant. Each page has a 'dot grid' graph paper layout. Good for drawing sketch maps an such.
Overall I really dig Field Notes brand notebooks. I'm happy they developed a notepad that can stand up to hard use. Such as survival situations.
The Field Notes Expedition Edition comes in a 3 pack and retails for $9.99.
Follow the link to purchase your own set: