There are so many sketch books and pads available for artists that it can be difficult to choose the best sketchbook for pencil drawing. I know for myself I have purchased many different brands and types of sketchbooks over the years. However, there is one brand that consistently provides a product that is reliable and a joy to work with for many artists, myself included. That is Strathmore.
Strathmore provides a numbered series of paper products with a specific colored cover that makes it easy to determine which product you are buying and for what purpose. In this post we are focused on graphite sketching, which means the paper will be thinner and lighter. The goal isn't to create finished drawings in this sketchbook, but to have something at hand to freely experiment in without feeling like you are wasting a precious resource.
There are a wide range of Strathmore papers for every level of artist. The naming reminds me of the old rhyme my grandmother used to tell me: "Good, better, best. Never let it rest, til the good is better and the better is the best." Here is a listing of the various series of paper they produce:

Designed for ages 5 and up, the paper types and features have been selected to enhance the creative process.

Good quality paper at a price that's economical enough for daily use. The broad range of papers is a great starting point for the beginning and developing artist.

An affordable line of art pads featuring extra high sheet counts and durable construction. Tear away fly sheets reveal a heavyweight, customizable blank cover made from high quality, steel-blue mixed media paper.

Better quality paper designed for quick studies and the practice of techniques with any media. Also suitable for final artwork by beginning or student artists.

Best quality with superior sheet formation, harder surfaces, and a variety of textures that work with a wider range of media and styles. Perfect for advanced artists.
The green cover sketchbooks indicate recycled, post-consumer fiber that is rated Grade A for cleanliness.

Premium quality, professional grade fine art papers. All papers are archival quality, acid free, and manufactured with cotton fiber for enhanced surface durability and exceptional results. Perfect for professional artists.
Recommendation: 400 Series Sketch
For general sketching practice on good quality paper, my recommendation as the best sketchbook for pencil drawing is the Strathmore 400 Series Sketch Book. It works well for any dry media such as graphite, charcoal, or pastel.
While you can use ink or markers, it will bleed through. This means you will be limited to using one side of each page. In addition, you would need to use some scrap paper beneath your drawing page to avoid damaging the pages underneath.

Many sizes available
You can find any size to fit your needs
There are various sizes available, but the size I recommend for general sketching is 9" x 12" with 100 pages. It is easy to carry around, yet still large enough to focus on details when necessary. The 60lb, acid free paper is thick enough to use both sides of the paper.

Perforated sheets
This feature is a must if you want to remove your sketch from the book to reference for finished work or even to give as a gift.
Ripping pages out of a spiral-bound notebook will leave you with jagged, ugly pages. It also leaves ripped paper in the spiral binding that is a pain to remove. Perforation is a must for any quality sketchbook.

Heavy chipboard backing
Unless you want to carry a drawing board around with you, the sketchbook needs to stand alone.
The heavy chipboard backing on this sketchbook will keep the paper upright and stiff while you are drawing on the go. The sketchbook is rather large, so without a good backing it would be difficult to draw straight lines.
Strathmore also provides a product finder tool on their website that can help you choose the right paper depending on multiple criteria, such as media, format and paper series. I'm sure that if you try Strathmore paper you won't be disappointed.