For a MTB?
I have all wheel sizes, a 26" wheeled Turner 5-Spot, a 27.5 " wheeled Turner Burner, and a 29" IBIS Ripley.
The 26" 5-Spot is fun and feels fast but is slow and is not nearly as confident in the rough stuff as the 29er. The Burner feels like the spot but with heavier wheels. Nothing is different between that bike and the 26" bike. The times on my local trails are similar but I feel slower. The Ripley is another story. Fast, confidence inspiring in the rough and feels like I am barely working. And almost as fun as the 5-Spot. These new 29ers are just out of this world.