A few people have mentioned it, but Jisho.org is a great dictionary online. I haven't found many others that I like for word dictionary. It's important to know that sometimes what you want to say in Japanese isn't just a single word though, but could be more a phrase or conjugation set up. You'll want to find a grammar bank to look through as well for more complicated sentences.
For English>Japanese, I like to use eow.alc.co.jp. It also contains numerous "sentence fragments", which can sometimes help choose the correct translation.
I personally use Japanese Dictionary Takoboto for looking up words. It's reliable and includes verb conjugation, example sentences, how to use words, etc. in definitions. You can also make word lists and do tests on them. I find the interface to also be very tidy and intuitive, and it has a grammar checker where you put a sentence in and an AI fixes any grammar mistakes. It dosen't do sentence translation, though. It has premade word lists, (JLPT, counters, numbers, months) but the topics are slightly limited in my opinion, however you can download other ones made by other users of the app. It also has a reader, a kanji finder (by radical), grammar, and the dictionary can be switched into a few other languages too. If you'd prefer something with more word lists, Shirabe Jisho has word lists for numbers, verbs, different verbs depending on verb ending, word lists from most common to least common, yojijukugo, adverbs, auxiliaries, adjectives (seperate ones for i/na adjectives) and more. I don't use it often so I'm not completely sure, but I think Shirabe Jisho lacks a lot of features of Japanese Dictionary Takoboto. As far as I know, you can't do tests, the verb conjugation forms exist but don't include explanations for each forms meaning which could be inconvenient, and it only does English if I'm not mistaken. However, you can still find kanji via radicals on it. I would recommend Takoboto more than Shirabe Jisho, I've been using it for the past 4 years and never had a complaint. I hope this helps!
Shirabe Jisho and Takoboto are useful for different things, imo. For example, Shirabe Jisho is good for some kinds of Shiritori because when you type a kana it only shows words that start with it, but Takoboto is useful for finding words that, say, end with a certain kana, because you can write question marks where you want random kana
