Obviously there are a lot to choose from and there will be different experiences based on target industry and discipline. B to B focus is a challenge that many groups try to maintain meaning you tend to find service providers and job seekers often.
In my experience in Denver and other markets, the best for what I know about you are
ACG and
IEEE. Both are national which could give you broader resources - both have great resources! Denver region specific -
CXO is the best B to B. They have tight criteria to maintain this so check the site and to see if you qualify. Check out CXOVP too.