Devil's dictionary

A person who helps you solve problems you'd never have without them.
(http://humorgags.blogspot.com)

One that teaches; especially : one whose occupation is to instruct.
(http://www.merriam-webster.com/dictionary)

Monolingual or bilingual? Which is more/most important?

Each of them is equaly important. Bilingual dictionaries are for lower level students, probably up to B1 or even B2 level. Monolingual are for those who are familiar with quite number of words and certain number of structures, those who are able to understand the definition (so B2 and up). I particulary like one which is the mixture of the two - it's  Oxford english-serbian Student's Dictionary. It is actually monolingual dictionary where at the end of every definition there is a Serbian equivalent of the word.