Any siege equipment for me - sieges are my thing really - this game revolves around them as far as I'm concerned - avoiding field battles and going for the cities is pretty much my strategy. So siege towers (even the silly overpowered ones), onagers - great in field battles too, the lowly battering ram is useful... the ultimate siege device (especially for wooden walled settlements) is the elephant - smashes open gates, knocks down walls, and in narrow streets? Nasty, especially with cavalry coming down the other way.