Well, its generally the class you are most comfortable with. For me its Demon Hunter because I played that the most hours and I like the ranged playstyle. He also has good speedrunning builds which are easy to gear for and to play.
The class you like is the best class for the most straightforward season stash reward. The season stash in Rocket League is a collection of various items that you can collect by playing the game and spending your in-game currency. Each item in the cache has a rarity assigned to it. The more rare the article, the rarer it is to receive