I know this: http://dictionary.cambridge.org/pt/dicionario/essential-british-english/

I like wordreference. :-)

I go to ahdictionary.com first. As a U.S. native speaker, ever since it came out I've always been very fond of the American Heritage dictionary. It is not a truly comprehensive unabridged dictionary, but it is quite a bit bigger than the average "desk dictionary." What I like about it is the very high quality of its definitions, which are thoughtful, clear, and precise... and the guidance in its occasional usage notes from its "usage panel," which deal with questions of style and usage (e.g.the word "ain't"), and are always excellent.

I think is oxford the best and dictionary. com too.

Мне нравится Яндекс переводчик именно как приложение для телефона:) удобен,не зависает и переводит так что смысл всё же понятен:)