What are the best CPM networks to use on my site?

In my experience, CPM ads pay much less than CPC. I think that your issue with AdSense was poor optimization / ad placement.

I have a client who had very low CPC with high traffic website, and was making well over $10000 per month with AdSense.

Before we optimized his site, with same traffic he was making less than $3000 (which is still pretty good).

There may be some good CPM networks out there, but overall, CPC should yield much higher income.

What was your RPM (revenue per 1000 impressions) with AdSense?

In my experience, the CPM model does not pay as much as the CPC and the CPA. However, if you are planning to use the CPM to achieve inner ad process inside your website it is feasible but it would require a large amount of impressions.

This would lead you to pay a lot for a small traffic.
My advice to you is to switch the model or should you remain in this one, you can go for Affiliates other than Google such as Kimia, Mobusi basically the affiliate marketing big guys.

If you have any further questions, I'd be happy to answer them over a phone call.

If engagement is low on your site, not many CPM networks are going to jump all over your traffic, at least not at good rates. That being said, it's about the niche that your site sits in, but if it doesn't, try to curtail that traffic to a niche to demand a better CPM. Those networks nowadays are getting more picky, and don't have the open door policy they once did.