The one that fits your family budget, weather and destination preferences. Truly, school calendars are pretty standardized - and airfare / airport crowds are a good indicator if that.
If your kids are out of school any of the ”shoulder season,” you can possibly miss some of the crowding. It is when WDW used to offer resort sales, free dining or lower-priced theme park tickets.
You can also do a broad search ”Find a Cruise” on the DCL website. The lowest priced cruises seem to fall in times when there is going to be lower demand >> fewer kids. That is actually when I look for a cruise.
A rough guideline if lower rates and crowds I have found over the years:
January - about week 2 into February (there is MLK holiday in there, gets your one day off school in many states).
April - about the last two weeks (after Easter) and much of May. The closer you get to June, prices start rising. Shocking to me, some states are still in school until the 3rd week of June. If yours lets out earlier, May/Early June may be the sweet spot. The air and water has already warmed up. And hurricane season doesn’t have months of warm water to create the powerful storms. (Caribbean cruises).
(Mid August to mid September). School starts are spread out. This is peak hurricane season, with a day around Sep 11-15 being the one most likely to experience a hurricane. September after Labor Day through mid-October have long been a big block of shoulder season. “Jersey Week“ for school break is the first week in October. Even our local school system now has a fall break that week.
The first week of November and the first two weeks of December are often lower airfares and cruise prices.
…The Caribbean can be really hot, even into December. The water temps do drop as you get farther away from summer, but temps don’t make significant changes throughout the year. (The Bahamas has very shallow water.).