They're all pretty comparable in terms of features, so I guess it depends on exactly what else you need from your webinar solution. How many people will be attending? What other interactive features do you want? What does your budget look like? Things like that.
Personally, I find WebEx a little difficult to use, though it might have improved in the year since I've used it. I would test out their free version before committing to the paid service.
GoToWebinar is pretty pricy compared to WebEx, though, so unless you plan on holding meetings with nearly 100 viewers, then it might not be the best option. Since ConferTel doesn't make their pricing readily available, I'd say that's a bit of a red flag in terms of how much it probably costs.