I always use the oxford dictionary.

Hi Sharkman:

 For   my  quick online searches,  I prefer "The Free Dictionary Online" by Farlex

<a href="http://search.yahoo.com/search?fr=mcafee&type=A010US0&p=free+farlex+dictionary">http://search.yahoo.com/search?fr=mcafee&type=A010US0&p=free+farlex+dictionary</a>;

That is a very convenient dictionary.  However,  if I have a serious wordsearch:

What is the OED?

The OED is one of the largest dictionaries in the world and the accepted authority on the evolution of the English language, tracing the use of more than 600,000 words over the last 1,000 years through 3 million quotations. The OED defines:

how a word has been usedwhere it came fromwhen it first entered the English languagehow its meaning has changed over time and around the world"

See:

<a href="http://www.oxforddictionaries.com/definition/english/free">http://www.oxforddictionaries.com/definition/english/free</a>;

   I do not care for some of the definitions in Merriam-Webster Dictionary.

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I you are thinking of purchasing a serious medium-sized printed desk dictionary I am <em>very</em> fond of the American Heritage Dictionary. It's now in its fourth edition, but I still use the third because a) I'm a cheapskate, and b) a high school classmate of mine worked on the third edition and he signed my copy.

The American Heritage is about as big as a dictionary can be and still be a usable desk dictionary. The definitions in it are the best I've seen in any dictionary. The little pictures and photographs are helpful and well chosen. And the usage notes are helpful and sensible.

In my opinion it is better than any of the Merriam-Webster desk dictionaries.

The Free Dictionary Online - I find it very helpful and I like it. You can hear the correct pronunciation of the words - both British and American. It sugests exemple sentences, as well as synonyms/antonyms. The latter I found invaluable while preparing for exams, especially for the writing tasks. I have even downloaded the app on my phone so I can access The Free Dictionary from anywhere! :-)

I agree with Bruce too. The Free Dictionary usually gives example sentences as well. Aside from that, it's useful to be aware of which regions use which dictionaries.

Oxford (OED) - UK

Merriam-Webster - USA

Macquarie - Australia

Normally, these dictionaries will point out regional differences.