Leverkusen didn't lose any game after 7 rounds and they have only one draw while Bayern got 2 draws and no defeat making Leverkusen in the first rank with 2 points difference from Bayern Munich.
I find Stuttgart interesting too although I doubt they could keep their position at the opposite of Leverkusen.
There is nothing impossible for Stuttgart to become champions in the Bundesliga this year, if I see that Stuttgart has been quite consistent in playing the last 7 matches. Even though they have experienced 1 defeat, apart from that, Stuttgart has a player who is quite consistent in his game, namely Serhou Guirassy as a striker, and has been the top scorer in the last 7 matches. I think Stuttgart will enliven the competition in the Bundesliga in fighting for the title this year, although it is still too early to draw conclusions now.
I think for the first time in over a decade, the German Bundesliga will be regarded as one of the most competitive and entertaining league among the top five leagues in Europe. Before, it was just a one-club show by Bayern Munich and sometimes Borussia Dortmund tries to give Bayern a little competition but this season has been absolutely different as four clubs in the are currently battling it out in the race for who win the German league title at the end of the season between Bayer Leverkusen, Stuttgart, Borussia Dortmund and Bayern Munich.
Bayern Munich is undoubtedly the strongest of them all when it comes quality of players in each of the clubs but Bayer Leverkusen wants to prove a strong point as they're still unbeaten in the league.