This is always a tough one so I'm opting out.
Rocky movies are more iconic, but I liked Total Recall, Predator, and Running Man just as much.
Between Conan, Demolition Man, Rambo, and Terminator, all the lesser of my fav movies of theirs, are on equal footing with each other.
There are so many other great ones, like Tango and Cash, Over the Top, Commando, Twins etc..
How can you choose? They made the 80's and 90's
If I had to distinguish them, I'd say Sly was better at Drama, while Arnold did sci-fi/fantasy better