What Duolingo language is the least similar to English
I'd like to learn a language with a very different grammar to English and I was wandering in your opinion what is the language (on duo and in general) that is the most different from English I've tried Chinese and the grammar was very different but also I found quite easy and logical I am going to continue with Chinese but I am also looking for a bit of a linguistic challenge in the meantime
Take a look at this table: http://www.effectivelanguagelearning.com/language-guide/language-difficulty
If "least similar" is correlated to difficulty then you can pick one of the Category V languages. If you tried Chinese and didn't find it that challenging then Japanese might be what you're looking for.
Almost certainly Klingon (which is not released yet, but hopefully will be pretty soon). Mark Okrand (the creator of Klingon) has stated that "a design principle of the Klingon language was dissimilarity to existing natural languages in general, and English in particular".
I think differences express themselves differently in various languages, so it depends exactly what you're after.
I'd recommend trying the languages that are not indo european. As well as those already suggested, there's Hebrew, Hungarian, Swahili, Vietnamese, and arguably High Valyrian, though it's hard to say, from a quick perusal of the Wiki page, how much it does or doesn't owe to European languages. I imagine you'll find something in there that will satisfy the differentness/difficulty desire!
Typically, the more different from your mother tongue/other languages you know well, the more difficult, at least in terms of unfamiliarity. (As you've discovered with Chinese, difference =/= inherent difficulty. I've discovered the same with Hebrew. Although it's very different from English, and there are relatively very few borrowed words and cognates, the grammar itself is fairly straightforward and helpful. I find it easier than Russian in many ways, although Russian is much more closely related to English.)