Excellent choices so far. I think we can all see that there were a
lot more than 10 defensive greats in NFL history. As to some of the names mentioned, I can say that Thomas, Nitschke, and Lane would have definitely made my top 20. I'm a particularly huge Derrick Thomas fan.
As to some of the others mentioned, Bednark was obviously a great big linebacker of his earlier era. However, for the same reason I moved Nitschke (and even Butkus) down the list, I didn't include Bednark because he didn't go against the physical OL specimens or chase the speedy RBs and WRs that Lambert, Ham, Taylor, and Lewis did.
As to Mills, I loved him in both the USFL and NFL. He really was a similar small, wicked smart ILB to Singletary. I chose Singletary, however, for his manning those winning Chicago defenses and his extra mean intensity. As you can see, I have completely forgiven him for the farcically terrible job he did coaching my 49'ers...