The best BB weight is determined on your play style and the limits at your playing locations. You should confirm what the velocity limit, joule limit, and whether biodegradable bbs are required are at the field you intend to play at before picking your bbs.
When choosing a bb weight keep in mind that the heavier the bb is the slower the velocity but greater the consistency. Lower weight bbs like .20 and .25 are good for close range battles where velocity means more than accuracy. Most AEGs perform best with .25-.32 gram bbs. Dmr and sniper rifles perform best with .32-.40 gram.
This chart gives a good estimate of the different bbs available https://www.valken.com/assets/download-files/AirsoftBBChart-v1.jpg
This video goes over everything you need to know about picking bbs