Which division is the toughest to win in?
Do you think the teams in this division will maintain high level competition moving forward?
I think most people are answering "which is the toughest division to win" which is not the same as the "toughest division to win in." The second is what was actually asked and isn't about the best teams in the division; rather, it is about the depth of the division. The best measure in my mind is to sum the division W-L records; since all intra-division games will cancel out (one winner, one loser), it means that the division with the highest division winning percentage beats the other divisions most frequently.
So far this year, that's the Midwest. They're 27 games over .500 as a division (the Central is just 17 games over). The Spurs have a lot to do with that, but the whole division is tough and even the back end of the division beats up on others somewhat (the Spurs lead the way at 16-2 but the Rockets are 18-7, Mavs are 18-9, Jazz are 16-6, even the Nuggets 10-14 against non-division opponents and the Kings are 8-16 outside their division).
I don't think the competition will remain high-level in the long run because we happen to be in a cycle where none of these teams are actively trying to tank... and that part of the cycle is going to come. Two years ago, who would have thought the Nets, Bullets, and Knicks would be actively tanking in the Atlantic? Every season, three or four teams start to go into a rebuild and - especially if the division remains tough - some of these Midwest teams are eventually going to decide it's better to build for the future than try to compete now.