100% honestly Calendly was the first one I tried a few years ago and I’ve used it since. I don’t have experience with the others. Although now I may give them a try to see if I can find a better bang for buck solution.
We were using Calendly for a while but it always felt too cold and transactional in nature, so built our own product zcal (https://zcal.co/) to make scheduling personal again. I know my answer to this question would be partial so, happy to hear any feedback at https://twitter.com/saurabhc135 🙂
We use calendly paid plan and get lot of extra features like integrations support, Workflows for calls and more. Did try MixMax calendar for short time, nothing much to talk about there, other than ability to integrate widget within the mail copy.
I think Calendly because that was the first scheduling app I was introduced to in 2018 and I kind of stuck to it without the need for finding other solutions as it perfectly suits my needs. This is clearly a situation where a user doesn't want to shift to another solution as there's no apparent burning need. Plus Calendly enjoys a first mover's advantage :) I haven't even graduated to the paid version!