Probably the weirdest - and also the bleakest - fiction I have read is Jonathan Bowden's disorientating novel
Kratos. The latter deals with insanity from a Nietzschean perspective.
http://www.jonathanbowden.co.uk/images/thumb_kts.jpg
And at the very high end of the scale, particularly with regard to literary skill, is Alexander Theroux's
Darconville's Cat. The novel reads like a completely misanthropic Hermann Meville, but what makes the it unusual is its very multilayered construction and the sheer baroqueness and density of the prose. Really extraordinary.
http://photo.goodreads.com/books/1172583713m/193408.jpg