I like using Artha as its way simple to use. Type in a word and you get the results fast. No need for scanning the internet. You also get some related words below the dictionary meaning as well. For everyday use, I use Artha.
GoldenDict is similar at the outset, but you'll soon realize how powerful it is. After installation, this is by default an online dictionary scanning Wikipedia. With GoldenDict, you will need to find an offline dictionary file to download and install (dont worry, they are free). Since my wife is Russian and going for her second masters, she uses GoldenDict as it smokes Artha away with its massive functionality. She is able to install tons of dictionaries. American, British, Russian, Ukrainian, French, Polish, etc. You can even install audio dictionaries. GoldenDict can handle a few different dictionary file types such as Babylon, StarDict, and ABBYY Lingvo dictionaries. If you need word power capabilities, then you definitely will fall in love with GoldenDict.
You can find numerous offline dictionaries in a variety of languages at the GoldenDict website: http://goldendict.org/dictionaries.php
Of course there are a few other dictionaries in the software center, but these are our preferred picks.