The most exposed single player Quake mod out there by far, and also thankfully the best and coolest. Between the levels that come out of the box to the still-growing number of third- (fourth?) party maps that have been made for it (including my own: google Threat Assessment on Quaddicted dot com, and look out for Azure Anesthetics "when it's done") you have every type of Quake experience imaginable, from tight gameplay-focused maps to massive adventures where you can spend hours combing for secrets. While I'm not in love with all of the additions, the amount of added content from monsters, items, and traps to under-the-hood mapper goodies is as impressive as it gets. But to me, the single most important change is converting the shotguns to projectile weapons, which raises the skill ceiling while also subtly upping their raw power (or at least making gibs come more often?). And when I thought it couldn't get any better, the Jump Mod's jump boots were added.