He won't be remembered as one of the greatest actors of all time, but when it comes to personality, charm, and charisma, he's in a class of his own. A true movie star in the best sense of the word.
The amount of cheesy one-liners that would make any other actor look stupid but made iconic by him speaks for itself.
Take Predator, for example. A serious action movie, not really meant to be humorous. He impales a guy and then says "Stick around". Any other actor says that and it would make your eyes roll back in your head. But when Arnold says it it's endearing.
If I had to pick my personal top 3 of his movies, it'll probably be: Predator, Terminator 2, Total Recall. Probably in that order.