When trying to learn Spanish, one of the most frequent topics of discussion is about what type of dictionary is best.
For the beginning learner, the answer is easy and obvious: any Spanish-English bilingual dictionary should suffice.
For the more advanced learner, however, the subtle -- and not so subtle -- differences in the many available bilingual dictionaries become more important -- but more about that in a minute.
First, I want to address the very mistaken idea that an advanced learner should not use a bilingual dictionary at all, but rather should use a "monolingual" Spanish dictionary (i.e. one that is entirely in Spanish). This of course is the type of dictionary that a native speaker would use. But guess what? You're NOT a native speaker. No matter how well you speak Spanish, your brain is "wired" in English. This means that there are certain concepts that you will grasp quicker, easier, and better in your own language. The advanced Spanish learner DOES need a monolingual Spanish dictionary, because certain questions about grammar and usage can only be resolved there, but in general a bilingual dictionary should be kept closer at hand.
To illustrate my point, suppose you are reading a passage that includes the word "roble" that you do not recognize. Consulting your monolingual Spanish dictionary (I'm using the one listed below...), you discover that a roble is a " Arbol de la familia de las fagaceas de hojas lobuladas y madera muy dura, cuyo fruta es la bellota, y que puede alcanzar hasta 40m de altura…” Now, I even did the work for you myself by looking the word up in the dictionary for you, but other than learning that a roble must be a species of tree, I bet you still don't really understand what it is.
Now look what happens when we consult our trusty BILINGUAL dictionary: we find the single word "oak." And there it is, instant understanding, short and sweet.
That's why you need the bilingual dictionary no matter how fluent you are in Spanish. There are some things that are just more easily explained and understood by using your native language.
So here are the two dictionaries that I personally think are best, and that are always with me:
1). The New World Spanish/English, English/Spanish Dictionary http://www.amazon.com/Spanish-English-Dictionary-Diccionario-espa%C3%B1ol/dp/0451181689/ref=pd_sim_b_8
2). Larousse diccionario Básico de la lengua española