They're all difficult to ride, but some more than others
Recognize the standard unicycle by its classic presentation. A single wheel, typically about 20 inches in diameter, with 5-inch pedal cranks (or thereabouts) is the favored locomotion of your garden variety rider or comic performer. If you get one unicycle, this will be it.
The mid-sized wheel creates enough control and speed to make it functional for experienced users but also great for learning. These guys are great for light tricks, but be warned, beginner unicycles aren't designed to undergo the same stress as a serious freestyle or trials unicycle. They may look alike, but you'll end up cracking a frame if you're not careful.
Trials Unicycles
Trials unicycles are designed for traversing complicated obstacle courses comprised mainly of common urban items such as railings, ledges and stairs. Naturally, the objective is to complete the course as fast as possible without touching the ground or at least breaking skin. As such, the frames are quite a bit stronger than beginner unicycles, but the wheels are usually the same 20 inches in diameter.
The Cruiser
This name is reserved for unicycles with a much larger wheel than a standard unicycle. While we say 20 inches is standard, to be a genuine cruiser the wheel should be 28 inches or larger. Thinking through the physics of gears and whatnot, you should conclude that with the same size cranks, a cruiser should move faster but require a little more effort to make a revolution.
Because of the larger wheel and greater potential for speed, the cruiser unicycle should never be considered a beginner's unicycle. If you're athletic, ambitious, and eventually want a cruiser, you may want to start out on one. Prepare yourself for a hard road, though. But once you get the hang of it, they are loads of fun and well worth the effort.
You can guess what these unicycles look like. They may or may not be spotted, but they always have a seat that is about 5 feet off the ground. The wheel is powered by a vertical bike chain that puts the pedals within reach of the rider's legs. If you're looking for a creative way to be the center of attention, make an entrance, or make a hilarious getaway, the giraffe unicycle is just the ticket. We highly recommend safety equipment and, preferably, a health insurance plan that covers accidents of the absurd. Of course, lots of practice and quick reflexes will probably do the trick.
The Mountain Unicycle
Also known as MUNI, mountain unicycling is a popular pastime for experienced riders. Hefty climbs should probably be left for apparati of two or more wheels, but a fairly level trail with some bumps and rocks to keep things interesting is fun riding. You'll find mountain unicycles in all sizes, but the most common are around 26 inches. This is enough to give good momentum, but small enough to retain greater control than a cruiser unicycle.
The cutting edge
If, for some incomprehensible reason, the unicycle just isn't challenging or unique enough for you, there are a few remaining options. Look into something called "the ultimate wheel." Yes, it is something of a trite name, with "ultimate" and "extreme" being rather tasteless buzz words in contemporary parlance. But the locomotion is truly unique, as you are actually pedaling the wheel itself and standing upright rather than on a seat. The pedals are fixed and mounted on a single steel spoke across the wheel's diameter. Just hop on and roll. We would have called it "The Tooth Cracker."
Similar to the ultimate wheel is a free-standing wheel, with platforms rather than pedals, known as the "Impossible Wheel". Locomotion requires a modest hill and a history of novelty thrill seeking. Stopping depends entirely on inertia or a well-timed jump. If the aforementioned transport was called the tooth cracker, this would be the "Dentist's Delight."
Finally, the supreme novelty, single wheeled transport is known as the monocycle. It is pedaled like a bicycle, features a comfortable seat that is safely low to the ground, and looks like something whipped up for a world's fair in the mid 19th century. You sit inside this giant bicycle wheel and pedal it around you. Expensive, hilarious, and undeniably fun, the only downside we could really see is that it would be hard to juggle while riding.