Debian testing:
Debian is the one distrobution that I have used that feels like a complete operating system. apt-get is great, Debian isn't in a hurry to get up to version 10.0 like some distros that were released after it, and the community is great (with a few notable exceptions from Turkey...).
I use the testing version because Debian seems behind the curve of new software, and the only way to stay truly up-to-date is to run testing. If there are bugs, I can help by letting the developers know.