There are two major, recent online dictionaries. One is verterbukh.org and the other is englishyiddishdictionary.com.
I've heard ideological gripes about both. On one side are those who complain that englishyiddishdictionary.com is full of all sorts of newfangled words but doesn't include a lot words that proste yidn (simple Jews) use in the streets. On the other side are those who complain that verterbukh.org is not comprehensive enough and includes lots of daytshmerisms (Germanisms) that should be avoided.
But I think it's also important to realize that before these were online dictionaries, they were printed dictionaries, with all the limitations that come with printed text.
Verterbukh.org was printed as a Yiddish-English dictionary, meaning that you use it to look up a Yiddish word and see what it means in English. This is obviously very useful if you're reading Yiddish texts and you need to look up what something means.
EnglishYiddishDictionary.com was printed as an English-Yiddish dictionary, meaning that you use it to look up an English word and see how to say it in Yiddish. This is obviously very useful if you're speaking or writing Yiddish.
And here's why that matters.
When you read Yiddish literature, you'll probably encounter a lot of daytshmerisms -- words that are ideologically shunned by many Yiddishists. SO, even though one might, given their particular ideology, not want to use daytshmerisms, it is still essential that you have access to a dictionary that includes them. Because when you encounter them in literature, you need somewhere to go to look them up. That's one of the great values of verterbukh.org.
On the other hand, if the goal of an English-Yiddish dictionary is to tell you what Yiddish words to use, then it makes sense that, within a particular Yiddishist ideology, one would not include many daytshmerisms. After all, if you don't want people to use daytshmerisms, then why instruct people in a dictionary to use them? So one of the great values of EnglishYiddishDictionary.com is that the words it instructs us to use are those that are in line with anti-daytshmerish ideology.
OF COURSE, all of this is totally flipped upside down and around and made meaningless when the dictionaries are digitized, as they have been, and are searchable in both directions, as they are.
Even though verterbukh.org was printed as a Yiddish to English dictionary, you can search for English words on the website and find Yiddish equivalents.
Even though EnglishYiddishDictionary.com was printed as an English to Yiddish dictionary, you can search for Yiddish words on the website and find English equivalents.
And here's why this matters.
If you are reading a book and you find a daytshmerish word that you don't know the meaning of, you may well not find it at EnglishYiddishDictionary.com, even though the latter can be searched in both directions. You would have to consult verterbukh.org.
On the other hand, if you're speaking or writing and you want to know how to say something in Yiddish, verterbukh.org will not be very useful for many of the more recent terminology that has had to be developed in the 21st century. For that, you need to use EnglishYiddishDictionary.com.
So which one should YOU use? I encourage you to consider why you're looking for an online Yiddish dictionary. Are you reading primarily older literature OR recent Chassidic literature? Go with verterbukh.org. Are you reading Yiddishist-oriented literature, or learning to speak in as comprehensive a way as possible? Then englishyiddishdictionary.com is the dictionary for you.
Or, like me, perhaps you'll find that you have need for both. I subscribe to both, as they tend to complement each other: what I don't find in one dictionary, I'm likely to find in the other.
One last thing: the other dictionary that I highly recommend, in addition to these two, is any free online German dictionary. If you're reading old newspaper articles from the early 20th century, many of them are full of SO MUCH GERMAN that's not at all mainstream Yiddish, and many of those words are not in either verterbukh.org or englishyiddishdictionary.com. So for such words, you would need to guess the German spelling and look them up in a German dictionary.
Again, which dictionary you find most useful really depends entirely on why you're using a dictionary and what kinds of things you're using Yiddish for. And depending on how you use Yiddish, you may find that you really need multiple dictionaries, not simply because "oh one might have a word the other doesn't" but because they are fundamentally different in their orientation and purpose.