What’s the best online mind mapping tool you’ve ever used?

I am searching for the best free online mind mapping tools. The one I have used for a long time is called GitMind(link removed by moderator). As an English learner, I use it to study vocabulary and organize essay outlines.

Here are some features of GitMind:
1) Offer various mind map templates to choose from.
2) Custom themes and styles
3) Input outline and generate a map
4) Share and collaborate
5) Free cloud storage

While I was a fan of XMind, and I have lots of maps saved in XMind. But GitMind does not support import from other tools. Please recommend some free online mind mapping tools. Thx:)