There are really only three licenses you should ever consider when making a new project in earnest: GPL if you want it to stay free forever, MIT if you don’t care. Put an L in front of GPL if your project is a library. The end.

Any CC license including CC0 looks fine on paper, and they are court-tested, but anyone with a legal department won’t risk dealing with one in the context of software, because CC licenses are for creative works and scientific research, not software. The main thing they’re missing is a warranty release.

The Unlicense feels like an earnest attempt to fill the void that CC0 fails to fill, but it isn’t a tested license. Everyone with a lawyer won’t touch it with a 10 foot pole because they don’t want to be the ones to find out how enforceable it really is. Besides, the only thing it gains you over the MIT is the ability to go uncredited. Which is nice feature; if people didn’t want this we wouldn’t have so many attempts to make a license that has it. But I feel like of all the features of a free software license one should be concerned about, explicit lack of credit is a pretty low-rung one.

Direct public domain insertion is good and effective, but is not global. Many places in the world have no formal legal system to do this (Germany is a famous example). PD dedication without a permissive fallback license makes your code completely unusable in these places. It’s exactly why the CC0 and Unlicense exist in the first place.

Every single other license is either a meme license not worth the toilet paper it’s written on, a weaker version of the GPL/MIT, or the GPL/MIT with extra steps.