If you are a buyer, you need to check them all out - see who has the traffic that converts.
Plugrush is a good choice, but works a little differently. You need to create plugs that are displayed on publishers websites. I presume that you know you need to match the plug to what people expect to see on the landing page and all that. Here are some alternatives to
PlugRush:
Ero-Advertising
ExoClick
AVA Traffic
If you really want to get your hands dirty, you should try out
JuicyAds where I recommend you hand pick the sites and ad-zones on those sites. Then compare how your creatives perform.
Good luck.