The creatures, let's start with them. They have such originality and both are still unique concepts. They look great, are both freightening in different ways and were well thought out and designed by geniuses of the industry.
The movies are of course not held up by the creatures alone. Arnold was the reason I watched the action sci-fi classic 'Predator' initially as I am a big Arnold fan and though I am more of a Predator fan than an Alien fan, I also loved Ridely's 'Alien' and Cameron's 'Aliens', which I only saw because of Cameron's 'Terminator', which also featured Arnold.