What are the best monolingual dictionaries of each language?

[pinkmonster]

For Greek the best and most known monolingual dictionary is Mpampiniotis' «Λεξικό της νέας ελληνικής γλώσσας» by far.
http://www.lexicon.gr/eshop/el/modern-g ... -/1--.html

[LadyGrey1986]

For Dutch, it is de "dikke van Dalen"

http://www.vandale.nl/opzoeken/woordenboek

[Samer]

For Arabic, Lisan Al Arab لسان العرب is very comprehensive and I like to use it.

المعجم الوسيط by the Academy of the Arabic Language in Cairo is also excellent and includes more modern usages.

I use both electronically through http://www.baheth.info/index.jsp and http://www.maajim.com/.

[Arnaud]

deleted

[galaxyrocker]

As far as I'm aware, for Irish there's only An Foclóir Beag. It was supposed to be a precursor to a bigger one, but I don't think the latter was ever released. It ca be found online here, and can still be found in print here.

[Keys]

For Hungarian, from publisher Akademiai:
MAGYAR ANGOL nagyszótár
HUNGARIAN ENGLISH great-word-list -> as literal interlinear word-for-word translation ;P
Here on Amazon

Edit: Whoops read too fast again, you mean a thesaurus, that one is also from publisher Akademiai:
Magyar Szinonimaszótár
(after above interlinear lesson you should be able to deduce this one by yourself )
Here on Amazon

[IronMike]

For Russian, hands down it is Ozhegov's Complete Dictionary of the Russian Language / С.И.Ожегов, ТОЛКОВЫЙ СЛОВАРЬ РУССКОГО ЯЗЫКА, available online at http://ozhegov.info/slovar/

[Chung]

For Hungarian, from publisher Akademiai:
MAGYAR ANGOL nagyszótár
HUNGARIAN ENGLISH great-word-list -> as literal interlinear word-for-word translation ;P
Here on Amazon

Edit: Whoops read too fast again, you mean a thesaurus, that one is also from publisher Akademiai:
Magyar Szinonimaszótár
(after above interlinear lesson you should be able to deduce this one by yourself )
Here on Amazon

I'd say that the best avaialble monolingual dictionary for Hungarian is Magyar értelmező kéziszótár since it also explains the meaning of words, offers some example sentences and provides hints to inflection. It'll set you back about $30 US (7600 Forints) before shipping and it's not available online as a free, searchable database like Duden is for German or Kielitoimiston sanakirja is for Finnish.

The predecessor of this Hungarian dictionary is a 6-volume set from the early 1960s “A magyar nyelv értelmező szótára” but it's hard to find now. The successor is now appearing in 18 volumes as “A magyar nyelv nagyszótára” and is expected to be finished by 2031. As of 2014 5 volumes have been released.

See also "Please add a monolingual dictionary" for more discussion.

[Lumilintu]

For Estonian, it's Eesti keele seletav sõnaraamat (EEKS) and Eesti keele õigekeelsussõnaraamat (ÕS). EEKS is good for explanations and synonyms, but it also has some grammatical information for each entry. ÕS is mainly meant for looking up the right spelling, however it also has grammatical information which sometimes is a bit more detailed than the EEKS one.
I also like ETY, the Eesti etümoloogiasõnaraamat. Unfortunately it's not really useful for anything academic as it lacks references and sources, but it's good enough to quickly look up common etymologies.

[tractor]

I only know of Spanish

Diccionario Real Academia Española.
Diccionario Maria Molliner.

I sometimes think that there are words in dictionaries that do not have much usability in real life street.

I don't like the Diccionario de la Real Academia Española (DRAE) very much. There are very few examples of how the words are used, and the definitions are not always clear. I think the Moliner is a much better dictionary. However, these days I rarely use paper dictionaries. I prefer electronic dictionaries. One good thing about the DRAE is that it can be accessed online for free. The Academy also publishes the Diccionario panhspánico de dudas, which I quite like. Both are available here:

• http://www.rae.es

Other Spanish monolingual dictionaries available online are

• http://www.diccionarios.com/ (VOX/Larousse)
• https://es.oxforddictionaries.com (Oxford Living Dictionaries)
• http://clave.smdiccionarios.com/app.php Diccionario CLAVE

I think the one at http://www.diccionarios.com is the Larousse Gran diccionario de la lengua española.