italki classroom doesn’t run smoothly and is sometimes really hard to get it to work properly. Skype is a good option... I believe italki should start including Zoom among the options.

I personally prefer Zoom, because their security level has really improved a lot lately. Zoom also doesn't keep and hold your data as much as some of their competitors do, Zoom's privacy policy is much better than e.g. Skype's. See here: https://zoom.us/privacy

I prefer Skype, sometimes Italki can have a lag in the conversation.

I prefer Zoom, especially with younger learners, but Skype works ok.

Definitely Skype which allows to see my student while we're working on a Google docs file, and also sharing files is incredibly fast and easy. I've no idea why but Italki classroom didn't work more than 3 times in a row! I just stopped bothering. Occasionally I use Hangouts/zoom when Skype has a bug.