Stallone made great first movies. It's sequels that he kinda sucked at.
I'd say it's a tie between First Blood and Rocky for his best performance for sure, with Rocky being a little better as a movie overall.
Tbh though, he squandered his ability as an actor chasing being an action star. First Blood was the last movie he made that had any real acting in it.
I'm not saying that he didn't do good action movies, he did, and he still kinda does. It's the acting part that he started phoning in. Even in the Rocky series, the second one was half-assed acting compared to the first, and it went schlocky after that. It's like he was doing a caricature or impression of Rocky rather than seriously acting.
Now, again, as an action star, he was top tier. He had the moves, the look, and just enough skill to make characters pop a little.