That depends on your needs, the destination, and your goals for a holiday.
Are you traveling alone, or with a family?
But, as for a direct answer to your question -- best season to travel is the season that you are prepared for.
(A little planning will help you have a "best time", without regard to the season itself.)
Among family and friends, there is always debate about when is the best season to travel. The fact is, the answer depends on a number of things. Where are you traveling to, what activities do you plan to do while there, and even when airline tickets are cheapest.
I also believe that the answer to this question depends on many factors and the country where you are going. Perhaps you want to see the New Year's fair in Romania, or the tulip parade in Holland, or the October Fest in Germany - there are many options, so adjust to your plans. In general, before any trip somewhere, it is better to familiarize yourself with the details of a given country in order to know in what season it will be most beautiful. Or a specific resort where you are going. Last summer my wife and I rested in the Ukrainian Carpathians on the advice of our friends - we impressed the fresh air with a riot of greenery for a long time! And at the end of the trip, we went on an excursion to Chernobyl https://chernobyl-visit.com/ru/ - this, of course, is a completely different vacation, but still amazing.