Philly.
We've played the sixers time and time again and every time we knock them out. Embiid is not a leader, he falls apart in the clutch, and few people on earth guard him better then Horford does.
Likewise Embiid moves like a sloth in quicksand and would have absolute nightmares trying to chase Porzingis around the perimeter.
Also the sixers have absolutely no wings who can guard the duo of Tatum/Brown, nor do they have any perimeter guys who can guard White and Holiday. 7 game series against Philly, I'm calling Boston in 5. Plus Embiid just came back from injury so his already poor conditioning is probably even worse at this point.
Heat IMHO are much more dangerous.
Butler is not only very capable of giving us problems on the offensive end, he's also capable of giving one of tatum/brown a lot of trouble defensively as well.
Bam for some reason has given the Celtics problems since the beginning of time, and he's strong and mobile enough to potentially give Porzingis some hassles defensively.
Herro is inconsistent, but he's another guy who can get hot and score 30 on any given night, and the Celtics have a history of getting torched by those #3/#4 guys on the roster.
Now even with all of the above I wouldn't be too worried about Miami...until they picked up Rozier. He was a huge pickup for them and gives them a 4th offensive option who could score 25-30 on any given night - and he's just starting to find his rythm with the team too. Terry has a history of making big time plays at bit time moments on both ends of the court and he's intimidated by nobody - I think he was a really underrated pickup by Miami and fits their culture perfectly. He's a guy who will play hard and leave 110% effort on the floor every night.
When it comes to end of games the Heat are now a lot like the Celtics - they have a lot of offensive versatility and a lot of guys they can run plays for, and it's going to be tough to keep a body on everybody. And then you have Spolestra's proven championship coaching.
More importantly the Heat are just dogs. They will always give it everything they have. They will never give up. They are tough, they are smart, they are physical, they are well coached. When it comes intangibles, they are as good as any team in the league.
If you want to get past Miami, you need to beat Miami. They won't beat themselves like Philly all too often do.