The best feature is its ease of use. I was able to understand the tool from the first moment I started using it and it helped me to create flowcharts in an easy way that my colleages can to understand.
Another feature I really like is that I can create flows in a very fast way. In less than an hour, I can have a very well-developed idea to be presented to my boss. This is a characteristic that I appreciate very much, since it has made me gain points in the relationship that I have with my superiors.
Last but not least, I would like to say that the free version is very functional, and particularly to me, it has helped me a lot, since I have done several projects without having to worry about the price. That is, I think it is one of the best tools in relation to price / quality, because the free service is truly 'freemium'.
In my particular top 10 applications (which I use in my day to day work), gliffy, has earned by own meridian, one of the top 5 positions.