Terminator, but not Terminator. Time for my long winded answer why.
Alien and Predator are essentially the same concept: hulking monster terrorizes people from the shadows. They're both basic monster movies, but because the slasher comes from space, it feels like it's something different. It's very hard to make a really good slasher film, but it's also the simplest genre to try and do right. The problem with the recent sequels is that they try too hard to do something crazy and complex with the lore, which defeats why those movies worked in the first place, or become too meta or self aware.
Terminator is a different animal. Because while you do have the hulking monster from the shadows, you also have time travel, family/fate, future war, 80s tech paranoia. It's incredible just how well the first two movies (especially the first one) managed to find the right balance of all of those things to make a movie that feels relatively basic. So in trying to re-do Terminator, it's far more complicated. It's both a matter of honing in on which element you believe is the most central to Terminator plus updating what the zeitgeist fear is. (Termy G wasn't necessarily wrong with the SkyNet App concept, it was the execution that was so unbelievably clumsy and embarrassing.) So I think Terminator would be the hardest to remake but also that it has the most overall potential in being remade/sequelized. HOWEVER, the key is remake. No call backs, no Arnie, no Hamilton, no Easter eggs. It's starting completely anew.
Also consider the Sarah Connor Chronicles where the Terminators were infiltrating society as normal people, learning sentience, etc. There are different avenues you can take with the Terminator idea that is far deeper and more interesting than what can be done with Alien or Predator. But all of the later films have focused purely on the surface elements of the franchise.