In the best of all worlds, I’d have complete access to the OED on my computer and mobile devices with all updates for a nominal price. This not being the best of all worlds, such a thing doesn’t exist, and the price for the OED web service is far from nominal. (Last I checked it was several hundred dollars a year.)
Lacking the OED, I’ve made due with Dictionary.com (free with premium upgrades, iOS/Android), and you know what? It’s pretty darn good.
Dictionary.com does what I need: not just standard definitions, but good definitions with synonyms, sample sentences, audible pronunciation, and word origins for many words. Their sources are various, and include the old Random House Dictionary, American Heritage, Harper Collins, and others. The word origins and historical samples for some entries seem deeper than those sources, which makes wonder if they’re deriving some content from the OED.
In any case, the apps are strong in the kind of features wordies like. It’s not all that often I need to look up a dictionary definition, so I use Dictionary.com more for noodling around and browsing, and it excels in this. They provide a word of the day, blog posts on unusual word topics, lists of trending words and recent searches, and various other ways to browse through content. A thesaurus is included, with synonyms available for each word, and voice search is built in.
The appeal of the system comes from the fact that the base package is free, but a fair amount of muscle can be laid on the bone if you buy some premiums. Ad-free, expansive sample sentences, idioms & phrases, grammar & tips, and various dictionary add-ons (large slang, science, medical, and rhyming dictionaries, including some art) are all available. If you use the in-app purchase bundles, you can probably unlock the entire thing for about $14, but the $5 premium version has most of what you’ll need.
My kids use it as their go-to dictionary for school assignments, and I like being amble to meander through this great language, hearing pronunciations of obscure words and learning useless facts about etymology. It’s already taught me that parageusia means “an abnormal or hallucinatory sense of taste,” derived from the Greek “geus,” meaning “taste.” [Cue Johnny Carson voice] “I did not know that.