It helps that Stallone wrote the movie for himself to star in.
What's most impressive is that he didn't get the part until filming started.
I'd like to add Adam Driver as Kylo Ren. He gives a good performance, but the best part for me was that he looks like a real person. One of my favorite parts of the Sequel Trilogy is when he first takes his helmet off in front of Rey. He's not a monster, he's not some male model, he's just some guy, who's immature, and depressed, and angry, a bit goofy and in over his head.
Ewan McGregor as Obi-Wan Kenobi. He channels a young Alec Guinness beautifully, and always looks like he's having a blast doing it.
According to the source material, the casting for Professor X and Wolverine wasn't great, but I'll be damned if Patrick Stewart and Hugh Jackman didn't OWN in those roles. (Professor X is supposed to be from Chicago, and Wolvie is supposed to be hairy and 5'3.")
Speaking of X-Men, it was a garbage movie, but Kelsey Grammar as Beast was perfect.