Collins Spanish Unabridged Dictionary, 8th Edition (Harpercollins Unabridged Dictionaries) 
The Collins Spanish Unabridged Dictionary is simply the best Spanish dictionary you can buy. Here's why:
More than 750,000 entries and translations. The Collins Spanish Unabridged Dictionary gives you comprehensive coverage of both Spanish and English and the most up-to-date business, political, and technical terms. Native Spanish and English speakers worked side by side to create a balanced treatment of both languages and to make authentic and appropriate translations.
Clear, helpful layout: This fully updated edition of the Collins Spanish Dictionary offers a fresh and easy-to-read color layout and special layouts that highlight idioms and key lexical structures both in Spanish and English. In addition, the most complex entries have been given a special layout to make lookup quicker and easier. Coupled with this is the addition of the latest words in both languages, which makes the Collins Spanish Unabridged Dictionary the most modern, accurate, and user-friendly Spanish-English dictionary available.
More colloquial usage than any other Spanish dictionary: With its emphasis on current Spanish and English, both written and spoken, including all areas of modern life and featuring regional usage, the dictionary gives you the edge in finding the correct translation.
Reviews
It's big enough and complete enough that when I pick it up, it's like a trip to the library. A big library. The Sterling or the Widner. I read a lot and I have yet to find a word that did not have a translation in it. It not only has translations, it also has very complete information about usage, idioms, levels of courtesy involved in using the expressions, and so forth.
It I had to think of one drawback, I would say that it is so much like a trip to the library that I find that I tend to let myself get lost in the "stacks". I start out to look up some word or phrase and end up spending a lot of time just browsing through it from page to page.
