We've been using Zoom for about a half a year and it's stability is unbeatable. We have all-hands on Fridays with 100+ participants. The free tier can easily handle the requirements, the only limitation is that max meeting length is 40 minutes. After that you can immediately restart the meeting, but the pro option is also very affordable. It also features screen sharing and whiteboard sharing out of the box. I also appreciate that it's not mandatory to register an account to attend a meeting.