26 Inch wheels.
This wheel size is most common, and most bikes have this wheel size. This size has been proven to be the perfect size for most riding, whether it be on the street or on the mountain. Most riders prefer 26 inch wheels over the other size wheels.
29 Inch wheels
This wheel size started a few years ago and has really caught on since. Also known as "Niners" or "Wagon Wheels", these wheels get more traction on the trail and more power output per pedal stroke. A lot of riders hoping to ride uphill fast ride 29ers. This may seem like a win-win for riders but it has its pitfalls as well. The bigger the wheel is, the stronger it has to be, and the size makes it heavier then a 26 inch wheel. Also weaving through tight trails or through the urban jungle is more difficult with big wheels. Some people are die hard 29 people, while others hate on them.
27.5 Inch wheels
The newest inovation in the world of wheels is 27.5 inch wheels. These wheels are more commonly known as "650b" wheels, because the rims are measured in millimeters, and in this case, 27.5 inch rims are 650 millimeters. The idea behind these wheels are to mix the bennefits of 26 and 29 inch wheels into one wheel. The traction and torque of a 29er with the weight and maneuverabitly of a 26 inch wheel.
here we see 26, 27.5, and 29 inch wheels
700cc Wheels
These wheels are built for road bikes and street bikes, and have thin rims for slick, fast rolling tires. 700cc wheels are not actually one strict size, and these wheels usually have a little number after the "cc" that describes the size of the wheel. These wheels can range anywhere from 26.5-29 inches. The main reason 700cc isn't called 650b or 29er is because they are made for road bikes and not mountain bikes.
20 Inch wheels
Twenty inch wheels are made for kids bikes, and BMX bikes. Kids bikes are for small children to ride, and kids bikes wheels can range from 18inches to 24inches in size. BMX bikes have 20inch wheels because these bikes are built for tricks and stylish riding. A smaller bike is needed for riding stylish and doing lots of tricks, so the wheels have to be smaller as well.
I have ridden all of these wheels sizes and I think I'm gonna stick with my 26 inch wheels. While I like 27.5, I think the originals are the best.