They say results don't lie, and if you want to know the best at the end of the season you simply look at the results. Right now, that means Ferrari and Mercedes are the best teams in Formula 1.
But after Mercedes' difficulties at the start of the season, losing two races it should have won, you have to ask whether such a well-resourced and set up team should be making those sorts of errors. You could also say Ferrari is now alone as the best, having won two out of two, but even then there are some reservations. Sometimes you have to look further down the grid and take into account other factors to decide who is the best, as the Bahrain Grand Prix demonstrated.
The old problems Mercedes has had with the softer tyre compounds seemed to return on the Bahrain weekend, particularly in qualifying. It was that problem that left its cars third and fourth on Saturday, and meant Mercedes was playing track position catch up on Sunday when the car was quick, because once the tyres lose that initial bite, things equal out.