It depends on if you want to focus on British English or American English. For British English, the Oxford English Dictionary is generally considered the standard, but for American English, the standards are Webster's Dictionary and the American Heritage Dictionary. For offline dictionaries, I am presuming you don't mean in book form, though. If you have an iphone or Android device, you can use the free Merriam-Webster dictionary app: http://www.merriam-webster.com/promos/mobile-app-promo.htm. Also, the American Heritage dictionary has apps for both it's full (expensive) versions and a smaller (desk) version for $1.99: http://itunes.apple.com/app/american-heritage-desk/id286269540?mt=8 Same with Oxford: http://itunes.apple.com/us/app/oxford-deluxe-ode-ote-powered/id307233030?mt=8 or https://play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.handmark.oxford.dictionaryofenglish.full&hl=en Dictionary.com isn't a bad way to go either: http://itunes.apple.com/us/app/dictionary.com-dictionary/id308750436?mt=8