Are you referring to a general, monolingual (i.e. English only) dictionary?
If so, the most comprehensive dictionary available is the Oxford English Dictionary (OED2). The yearly online subscription rate is $295, the printed version (20 volumes plus 3 supplements) will set you back about $900. As far as I understand the online version gets updated every 3 month, and already contains some contents of the upcoming OED3 (due in 2030 or thereabouts). See http://www.oed.com
If you are looking for something more affordable, try Webster's 3rd International. Online subscription is $29.95 per annum, or buy the dead tree version for about $90. The online version offers pronunciation audio recordings for most words. They have a CD-version as well. See http://unabridged.merriam-webster.com
For a comprehensive dictionary with an Australian flavor, try http://www.macquariedictionary.com.au
. An annual online subscription is Aus$ 9.95.
Free online alternatives are dictionary.com (which provides the contents of the American Heritage Dictionary, 4th edition; AHD4) and http://www.m-w.com/
which offers the contents of Merriam-Webster's Collegiate Dictionary. The print version of the AHD4 is about $40. Size-wise it is more of a collegiate dictionary.
If you are looking for a printed bilingual English<->German dictionary, I am reasonably satisfied with my "Harper Collins German Unabridged Dictionary". I think the same book is sold in Germany under the name PONS. About $40 for the print version. The competition is the "Oxford Duden German Dictionary" which is available in print for about $35. I don't own it, so I don't have an opinion about it.