Today, in the opinion of many, HackerRank's calendar is the most complete one since you can find almost any competition, and its always up to date for a few months ahead. However, my own personal problem with it is that it's not sound. In other words, it has too many competitions that I usually do not have time to consider anymore, which ends up with a lot of noise on own personal Google calendar.
So in my attempts to create a more personalized calendar I found a bunch of feature requests for Google calendar to support filtering, or hide full day events, but these features are not supported, not even in the labs.
Then I found icalfilter.com which does exactly what I needed. I can give it the link to HackerRank's .ics and define some regexes for the events I want to know about like
/div.*/ and it will provide me with a link to a filtered .ics, which you can then add to Google calendar.
Here's a link to a filtered calendar (like the one I am using now, example above). It includes TopCoder SRMs, Codeforces Educational and regular Div 1 & 2 Rounds. You can use that as it is, or go ahead to icalfilter.com and create your own filtered version of any calendar you want!
Cheers and Happy Competing!