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.