TLDR: casale, Lijit, technorati are working best for me.
This was incredibly damaging to my reputation. I narrowed it down to Lijit after toggling my ad networks and informed them of the issue. I emailed them, explained how my users are getting these redirects on a daily basis, how this is unacceptable, and how I'll have to pull their ads until they're able to resolve the issue. Their response? They didn't care, they just said, we understand your concern for not wanting to continue our relationship. That was it, they're not looking into it, they didn't apologize, nothing.
I tried them again last month, after I cut them for the previous 6 months. Within a day, I had over a dozen users sending reports about malicious redirects. I disabled Lijit, and things were fine. I used Lijit for 8 months without trouble, and enjoyed their service since the interface is decent, rates competitive, tracking and payments done right, etc. However, since these issues started, I have no choice but to leave. It's sad they have no interest in solving them, because like I said, they have potential. Redirecting my visitors to malware though, and allowing that on the network? It's inexcusable.