I usually use two online English dictionaries: Dictionary.com and Merriam-Webster.com. In this article I will compare them. In short, I prefer Dictionary.com over MW.
Both sites have various appealing English learning materials including videos, games and illustrations. I will only talk about word definitions and not detail on those.
Last year, I read George Orwell's novel 1984. I used MW's Android app to look up words. The good thing is that you do not need an Internet connection to look up word definitions, though listening the pronunciation does require one.
Nonetheless, I found that some words were still puzzling to me even after I looked up them up in MW dictionary. Then I opened the Android app of Dictionary.com. This app requires an Internet connection. Generally, the results are much better. Dictionary.com provides more definitions than MW, and what I really love is that it gives you the discipline on which a specific definition is.
I think MW is more like a learner's dictionary. It uses less difficult words in the definitions. On the other hand, the editors at Dictionary.com don't seem to be picking easy words: they just use what they think fit most.
I once mistakenly thought Dictionary.com used American Heritage Dictionary as its primary source. It maybe true before. Until on the first days of 2013, I found the footnote at the primary definition in Dictionary.com was 'Random House Dictionary'. I sometimes simply refers the site as AHD. I must have confused the primary source of FreeDictionary.com with Dictionary.com. FreeDictionary.com uses the definitions from AHD, and I also find them of high quality.
The other day I happened to be at the class of a teacher from the biggest English training school in China. He recommended to his GRE class Merriam-Webster dictionary. I become puzzled. MW's definitions are not complete, and I do not think they can meet the needs of the students who are preparing for GRE, one of the hardest English tests which people in China take. I asked him which MW dictionary he was exactly recommending. He told me what he referred was the unabridged version. OK, puzzle cleared.
The gratuitous definitions we get from MW's website and its mobile apps are not from the unabridged version. To look up definitions in the unabridged version, you have to subscribe to the website: the annual fee is US $29.95 for the time I write this.
So for serious English learners, I recommend Dictionary.com over Merriam-Webster.com. If you want to use MW, since it's said that ETS uses MW as the authoritative dictionary, you really should get a copy of the unabridged version.