Skype for Business became Teams. Teams is the market leader in the business world with 270 million active users.
I’ve never heard of Teamspeak. I do use Discord but it isn’t great. I’ve only used Zoom once and the livechat didn’t work. My kids’ school used it for parents’ evenings during the pandemic but dropped it after a few months due to reliability issues. They moved over to Teams instead. Skype has an even bigger active userbase than Teams (300 million use it per month and 4 billion registered).
Skype’s the one I use to keep in touch with family overseas and my local GP surgery uses it for video consultations (I don’t have any experience of those).