Watch Time is the most important according to YouTube with probably Audience Retention coming in second.
You'll want to see growth each month in your Watch Time and Per Quarter! Go to your Analytics and look at "This Quarter", and see if you are expected to end the quarter with greater percentages than the previous quarter. Right now I have 25% more watch time this quarter compared to last quarter, and 25% more views with a month and half left in the quarter!
Essentially, the more Watch Time you have per month, the better rankings you are going to get from YouTube. The better rankings, the more views, and the more people who will find your videos.
Unfortunately that means that there are plenty of people who spam out videos to increase watch time, so it's even more important to release content on a more regular basis. 1 video a week isn't going to cut it, unless you've already got a very large audience.
At least that's how I see things.
Edit: But just to note that YouTube did also say that their algorithm punishes content that no one watches, so that's where audience retention comes into play.