I've been promoting CPA offers for 15 years.
In my opinion, there is no best network for them. You just need to sign up to many and see what offers they have and test them. A lot of times they will also have private offers, so check on this as well.
Be sure to split test offers that are the same type, but different advertisers. I have seen huge difference in conversion rates doing this. This is key to getting the best EPC's. Even test the same offer between networks, as sometimes you find one that produces better.
Also, networks may be given a monthly budget(limiting how many leads for the month can be produced) by the advertiser or the network may set caps on how many leads you can produce. So it's good to have the same offer that is performing for you from different networks, so if the the budget or your cap gets hit, you can switch to another network and continue promoting it.
I will tell you that I favor PPL (pay per lead) offers. This is because there is no credit card required. So conversions tend to be higher than offers that require a sale. All I need is for a user to fill out a form to get paid.
When it comes to specific offers, I favor offers that have some financial positioning...have a make, get or save money angle to them. That's what has overall worked best for me. Also, they tend to have greater mass appeal, so the potential to produce high volume exists.
Regardless what you promote, getting traffic will be key. I have always generated traffic using email, by making target data acquisition deals. That way I'm building assets and have the ability to generate traffic over and over, instead of buying traffic that will result in one-time clicks.
Anyway, good luck and stick with it. The potential to make 6-7 figures truly exists, once you have the right strategy and scale.