I think you're going to find everyone will tell you something different lol.. I'd say Devil blaster .2's are the standard. Saying that, a lot of people use Devil Blaster .25's as it's a great weight option for AEG's (Me included) depending on what gun you use, you'll try different brands, sniper rifles for example .43g Nuprols seems to be a fan favorite (Again, me included).
If you're after "The best quality", I'm sure other people will jump in with niche' brands, What I will say is I've used a pack of the Tokyo Marui .28 Perfect hit BB's, they're bloody amazing! You can tell they're super polished as it's a nightmare picking up any loose balls lol