As an RPG product line I think Lamentations of the Flame Princess has the best writing. Yes I admit that art and layout factor in to why I love Lamentations of the Flame Princess, but when it boils down to it the writing is what has me hooked. +James Raggi, +Zak Sabbath, +Rafael Chandler, +Patrick Stuart, +Kelvin Green, +Kiel Chenier have all written adventures (or supplements) that have had me rave (in the good way I assure you). These folks have style in spades. Their prose is engaging and *gasp* useful as well.
I do want to give a shout out to +Venger Satanis as well. His books are really well written too. His tone and references are always on point.