The Rhodia Pad
I love Rhodia Pads. A Rhodia No. 11 (A7, 3" x 4") keeps track of my to-do items and my shopping list, fitting nicely in my pocket, slightly smaller than my wallet. A Rhodia No. 16 (A5, 5.8 x 8.3") is about the same size as most paperback books, perfect for carrying around for smaller jobs. A Rhodia No. 18 (A4, 8.3" x 11.7") is the full-size pad that I pull out for long pages of notes, large sketches, or long mathematical models. I love that Rhodia pads are perforated--as a perfectionist, I'm often rewriting pages and storing them in a pocket file.
But Rhodia pads are expensive. Sometimes I have a really rough sketch or some random math to work through, and it doesn't warrant a nice sheet of Rhodia vellum. I also journal a lot. Rhodia web notebooks are lovely, thick notebooks, but they are also pricey. I also don't like overly thick notebooks for daily carry, because if I were to lose 6-12 months of writing, I'd be devastated.
Apica CD Notebooks
For a cheaper alternative with amazing paper, I use the Apica CD line of notebooks. The paper is great with all of my fountain pen inks, none of which feather in the slightest. They don't offer quadruled paper, nor do they have perforated sheets. They are nicely bound, come in a variety of cover colors, and cost under $2 each.