If looking strictly at numbers at the bottom I would give the edge to the ACC as having the most deadweight, but in terms of percentage then it'd have to be the Pac-12 due to fewer teams - especially if you were to remove USC and UCLA from the equation since they are leaving. Both conference have 5 teams that rated worse than the bottom-rated team in the Big 12 last year as per Massey Composite. Big Ten does have some deadweight, but a lower overall percentage as compared to their overall membership. Here's the bottom half of each conference last year (w/ new conference membership):

Big 12:
52 Oklahoma St
56 Kansas
58 Houston
59 BYU
66 Iowa St
67 West Virginia

Pac 12:
71 Arizona
79 Cal
92 Stanford
95 Arizona St
118 Colorado

ACC:
40 Duke
50 Syracuse
75 Georgia Tech
84 Miami
97 Virginia
104 Virginia Tech
106 Boston College

Big Ten:
31 Maryland
45 Purdue
46 Wisconsin
64 Michigan St
82 Indiana
87 Nebraska
100 Rutgers
114 Northwestern