My own experience has left me with a 26 off roader and a 29 road bike. The theory about the larger wheel rolling better seems to me to be correct, but the type of off roading I do, bridle ways, farm tracks, woodland paths, seem better suited to the smaller, more agile wheel. I can however see the advantage, in some circumstances, of using the bigger wheel off road, for cycle cross perhaps. The 27.5 would be a compromise for somebody wanting to use a bike for both road and off road riding. IMHO none are "the best" and none are "the worst". Its horses for courses. Its a matter of what suits you and your riding needs.