My portfolio (for taking notes, not my professional portfolio) has seen better days. I like the Nava Design Saffiano pictured below. But before purchasing anything, I like to know as many options as possible. What do you like?
I just use a moleskin. The slightly larger format. I have 2, a ruled reporter fold for to-do lists, and a graph paper version for notes and sketches. This plus iPhone is all I need. One of the VPs I used to work for at my former employer only used a small format moleskin. I loved when we would go to big meetings and all the business types would have their mega Franklin Covey type leather books, an he would just have a tiny notebook and a pencil.
ditto on the moleskin…
The Moleskine is great but I prefer the high capital cost/low disposable cost strategy v. zero-low capital cost/high disposable cost strategy.
I have a similar NAVA…it gets use only when going outside the office for vendor visits, etc.
A recent trip to Paperhaus here in Seattle found me purchasing some cool portfolios from Lost Luggage/Case Envy. Most of their stuff is sold on the Paperhaus website. You can go ‘cheap’ with the Light series or seriously upscale with the Ice Nine series. Installing screw extenders and extra paper will make your sketchbook/portfolio about 1" thick. You are locked in to their paper and supplies, but you could do a lot worse.
For one, I’ve never found the Moleskines to work for me, preferring a large/full format sketchbook which is completely disorganized, full of random stuff, fun to look at but utterly un-pro. Maybe it doesn’t work that well. Anyway - you can spend hundreds on the Lost Luggage stuff if high-capital is what you are after.
Pantone notebook, end of discussion. The spiral binding allows you to open it flat, works well with my Hi-Tec-C pens, and the Pantone label lets people know what a design snob I am. 
I’m down with these guys of late:
I use the Myndology disc bound “bare journal” http://www.myndology.com/disc.php.
See, I have this problem. I can’t put a pen to a fresh, new, flawless notepad like the Moleskine 
With the Bare Journal, you can take pages out, put new ones in, mix papers, and paper types(they have business card sized sheets too). So I feel a bit better.
Now if I could only learn to draw