Shimano's Ultegra group is one of the most popular out there with good reason: it's the perennial favorite when you factor the performance-to-dollars equation. With different demands for drivetrain ranges required for road, gravel, and CX riding, Shimano has created cassette ranges designed to allow the R8000 drivetrain to easily satisfy the needs for a variety of riders. The Ultegra CS-R8000 Cassette ranges as high as 32t, so you can set up the new Ultegra (or Dura-Ace R9100) with the range you need for gravel epics, 4 corner crits, hill climbs, junior nationals, and everything in-between.
Shimano's experience in forging, machining, and engineering allows for an enormous amount of technology that goes into tooth design, shaping, and shifting ramps. The cogs themselves are constructed from a nickel-plated steel that prevents corrosion and is slow to wear. Ultimately this leads to cassettes that shift smoother and more precisely—and last longer—at a price that's friendly to the self-sponsored racer.
The top five cogs are grouped as three and two, respectively, and are riveted to anodized aluminum spiders that add stiffness to improve shifting. The remaining six cogs are individual with spacers between, and an anodized aluminum lockring screws into the freehub to hold the entire assembly together. True to Shimano's Japanese culture—which has a name, poka-yoke, for built-in preventions of user error—one spline of the cassette is wider than the others and corresponds with the splines on the freehub, so the cassette cogs can only be installed with the right clocking of the shifting ramps, timing them for perfect chain pick-up and release.
Shimano Ultegra cassettes shift so well and are so unnoticeable that even World Tour teams will often sub Ultegra cassettes to get the bikes to the minimum weight limit. Frames and wheels are just so light these days, and adding mass to the cassettes isn't too detrimental to performance, and the Ultegra units shift on par with Dura Ace while actually lasting much longer.
Please note that the new Ultegra RD-R8000 Shadow Rear Derailleur comes in two sizes, short and long cage. Cassettes up to 11-30t are compatible with the short version, while 30t cogs and up require the longer model.