Depends how you determine best. If we judge managers by the impact/added value they bring to a team. Now. As in this season. Then Carlos Carvalhal takes some beating. 8 games with Swansea, 4 wins 3 draws and a loss. Previous 8 games for Swansea, 1 win, 2 draws, 5 losses. Swansea looked certainties for the drop. The way he has them playing I reckon they have more chance of climbing away from the bottom than we do.
If you're thinking the previous top level jobs and trophies determine the best manager, then fair enough. However Rafa has played a Carvalhal team four times in the last couple of seasons: lost three and drawn one. Not exactly evidence of outclassing him.