A matter of protocol
What's the best way to choose among those options? Your mileage may vary, but my own algorithm depends on the nature of the video call.
Scheduled calls: If you're planning recurring calls with your colleagues, the best advice I can give is to experiment, because what works for one pair of callers might fail for the next. Agree in advance that you'll use Skype for the first call, Google+ Hangouts for the next, and so on. Regardless of which service you choose for a given call, have a backup plan — if the call starts stuttering or stalling, all the participants should know which provider they'll switch to, or whether to try a conventional phone call.
Impromptu calls to a colleague: If I want to make an unscheduled video call to a colleague, my first step is to check the usual candidates to see if the person is online — Skype and Google+ Hangouts show participants's status, as does Messages for people in my Buddy List. (The status might be incorrect, but it's a good start.) If I can't ascertain a contact's online status with one of these services and I know the person is an Apple user, I try FaceTime. If none of those methods works, I use email, an instant message, or a phone call to see what works for the other person.
First-time video calls: First-time calls are the trickiest, because most people feel less comfortable chatting with strangers by video than in a phone call. If the person lists a certain service (such as Skype or AIM) on a business card, letterhead, or Web site, that's a fairly safe bet. Even so, the best idea is to give the other person a heads up first via email.
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