I have been a "student" of Russian for almost four decades, practicing the "osmosis" method of learning: I surround myself with Russian grammar books and dictionaries which languish unopened for months on end, in the hope that their contents will gradually percolate into my brain thanks to their proximity. Of course, it doesn't work and I am forever returning to square one.
The most ambitious dictionary I owned up to very recently is the Oxford Pocket Russian Dictionary. This is a paperback book which is quite good in its contents, not bad and quite up to date, but it is far too big for anyone's pocket and falls apart very easily. The Concise Oxford Russian is a much more complete tome, not much more expensive than the Pocket version, and far more robust (hardback). The only downsides are (1) the last version is about 10 years old (so some of the latest internet jargon will be missing); (2) it is rather too heavy and bulky to lug around to classes, as I discovered when I recently took it to Riga where I spent two weeks studying the language. And a lot of room in the suitcase!
Unless you are a really advanced Russian learner, I highly recommend the The Oxford Starter Russian Dictionary (Oxford Starter Dictionaries) [Paperback]. This is not very large, but for beginners and intermediates it is very good indeed, with lots of examples of practical examples of expressions you can actually use, and a table at the end showing 20 Russian declension forms....it is worth it for this one feature alone. Buy this before considering other dictionaries!