As I've said for years, you generally need a top 5 player to win championships, but even deep playoff runs almost always require at least a top 10 player.
There are only 4 teams in the East that arguably have a top 10 player: Milwaukee, Boston, Philadelphia, and Miami. To be clear, I do not think Butler is a top 10 player, but he is at least arguably one so I've included the Heat. The Sixers have a clear top 10 player and arguably another. Milwaukee and Boston both have clear top 10 players, though Milwaukee is the only team in the East with a without question top 3 player in the world. That is why I favor Milwaukee to come out of the East and also why no other team (except for Philly) even scares me a little bit. Boston can ride the Tatum train because the East just doesn't have many players like Tatum in the conference (why I was really happy to see Durant and Irving go West).
That is also why I think the West is far more up in the air as there are 9 teams that at least arguably have a top 10 player (Den, Mem, Pho, GS, LAC, Dal, LAL, NOP, POR). Now obviously not all those teams have top 10 players because at best only 7 do (since there are at least 3 in the east), but there is a lot more top end talent in the West. That stuff matters a lot when trying to figure out who can make a run to the championship or even just make the conference finals.