Finally got ahold of the BIG One: Schwalbe Racing Ralph 29 x 2.40. This is the highest volume and tallest 29er tire on the market. Tread width virtually the same as the widest available. Though it is not the most aggressive tread, it is plenty knobby and the tread is noticeably larger than the 2.25 version. All this and it still weighs just 600g.

Brief specs below. Full specs on the Schwalbe and 29er pages. Rim width comparison with seven other tires.