If you want a complete mirror of Wikipedia (including images), Wiktionari, Wikiquote, etc full HTML formatting intact that will download in aprox 30 hours, you should use:
English Wikipedia has a lot of data. There are 13.9+ million pages with 20.0+ GB of text, as well as 3.7+ million thumbnails.
Setting all this up on your computer will not be a quick process...
The import itself will require 80GB of disk space and five hours
processing time for the text version. If you want images as well, the
numbers increase to 100GB of disk space and 30 hours of processing
time. However, when you are done, you will have a complete, recent
copy of English Wikipedia with images that can fit on a 128GB SD card.
But the offline version is very much like the online version, includes photos etc:
(I tested the bellow article completely offline)
Other notable projects beside Xowa, Wikitaxi and Kiwix mentioned already, are:
Aard Dictionary is a free software dictionary with binaries for Windows, Linux, Mac and Android. Can build its own offline data from Wikimedia dumps or XDXF files with the help of Aard Tools(Aard Tools needs linux/unix and pyhton 2.7). Aard Dictionary has available many of the Wikimedia projects in many languages already pre-built but also other dictionaries like WordNet, etc.
Aard Dictionary doesn't need to index as
Kiwix does when you first load an offline data file, also is much
faster, and it searches in all the available dictionaries at the same
time displaying all the matching term in separate tabs.
Okawix which works on Windows, Linux, Mac and Android. Cannot build its own offline data but has available many of the Wikimedia projects in many languages already pre-built.
FastWiki which works on Windows, Linux and Android. Can build its own offline data from dumps or ZIM files.