I would say Terminator 2 by far, but since its precluded in this poll, I have to say Predator. Predator is actually a really good movie, it's got kind of a short story Most Dangerous Game feel to it (a classic short story). Commando is great for how over the top it is. He shows just how easy it is to kill an entire island worth of Latinos. And Terminator is certainly an important movie, but I just can't appreciate the special effects in it. I admit I've been spoiled by modern special effects.
Speaking of special effects though, I think they reached their peak with Terminator 2, in terms of quality and tasteful usage. Even today, everything in that movie looks damned good. And the most important part is that it is not over the top with CGI like modern movies. These producers nowadays need to learn a thing or two about restraint.