Probably Debian or Ubuntu. The question is...why do you care that much?
I've upgraded Debian stable (both pure and with some cherry-picked backports) and Ubuntu (non-LTS and LTS) systems in place and rarely broken anything, for years and years. When stuff has broken it's been a quick google and then slapping myself for not having read the upgrade guide.
I do generally wait about 2-3 weeks before upgrading, giving time for them to catch stuff that was missed until the great masses were set loose on it.
I've upgraded Debian stable (both pure and with some cherry-picked backports) and Ubuntu (non-LTS and LTS) systems in place and rarely broken anything, for years and years. When stuff has broken it's been a quick google and then slapping myself for not having read the upgrade guide.
I do generally wait about 2-3 weeks before upgrading, giving time for them to catch stuff that was missed until the great masses were set loose on it.