Depends. For me, agglutinative language...mandarin, basque, finnish etc.

But, latin languages have a complex grammar...a lot of annoying rules. Portuguese grammar...it's so annoying my god XD, there are a lots of rules and rules rules rules.

It 's my mother language, Vietnamese :D

Turkish! :( 

I think it's relative to the languages you already know. For someone who knows any of the languages that are off springs of latin, like German, French etc. it will be easier to pick up any of the languages that are part of that "language family". 

Independt of those factors Mandarin as well as Japanese have to rank high when it comes to the toughest languages to learn. Not because of difficulty per se, but the time needed to learn all the written signs. 

germany...