Pick your wheel size
"Standard" Mountain bike wheel size. This is still a very common size but it is not a fast rolling as a 27.5 or 29er. The 26" wheel is a lot more agile and easier to throw around. They tend to be a stronger construction as they are smaller.
27.5” Wheel (650b)
Sometimes referred to as 650b, this is a relatively new size wheel to the mountain bike market. They are much faster rolling than a 26” wheel and will deal with bumps better. The 27.5 (650B) product range has seen a steady climb in sales over the last year or more.
The larger wheel diameter offers a larger contact patch. In other words, all things being equal, a 29-inch wheel will put more rubber on the ground at once. This means that you’ll have more available grip for cornering, braking, and climbing. But for the most part 29ers are not as agile as the 27.5 or the 26”