Having played much more of A18 since I posted the above, I want to change my selection.My favourite has got to be the spear.
I rank the spear up there with A17's bicycle and A16's arrows sticking in targets in terms of "small addition that vastly improves the feel of the game".
I'm not the sort of person who obsesses over DPS and the like. The spear just feels so great that I'm happy to use it even if it isn't the most efficient weapon in the game.
(Having said that, the knuckle dusters are a close second when it comes to the feel of combat - both are great additions to the game.)