Definitely second Burtsplurt's suggestion of The Outsider. Excellent book. I think to a certain extent I found it to be what Catcher in the Rye is for many others. I was never a big fan of the latter, possibly because Meursault got there first with me. On your other suggestions, I've not read The Castle by Kafka, but I did enjoy both Metamorphosis and The Trial.

I'm just peering at my bookshelves now trying to settle on one or two individual books but I'm struggling to single them out. As I'm forcing myself:

The Remains of the Day by Kazuo Ishiguro is one of my absolute favourite books. It presents a wonderful portrait of a man evaluating his past, present and future, all of which is told with a startlingly convincing narrative. I was recommended it by an English teacher in college, and it remains one of my favourites.

Jane Eyre. I realise I probably lose masculinity points for picking a novel by one of the Brontė sisters, but it was a book that really took me by surprise. Really enjoyed it from start to finish, and I think made me reconsider both what my own literary tastes were, and what I actually enjoyed novels for.

A book I've read recently, A History of the World in 100 Objects by Neil McGregor I found to be quite thought provoking, and made me reevaluate my view of history, and the relationship between the past and the present, and of society's interaction with history. Some of the choices (all of which are objects in the British Museum) are quite novel and don't present the 'Kings and Queens' narrative of history.

The Ragged Trousered Philanthropists (Robert Tressel) was a book which helped to shape my political beliefs in many ways, and something which I think spurred me to get involved with politics whilst at university. While I know it's rather unashamedly a piece of propaganda, it's well written, and provides a very human portrait of the living standards of working men in19th century Britain.

Honourable mentions to Catch-22, HHGttG, and Norwegian Wood (Haruki Murakami)which are among the few books I've read more than once.