I think Invasion fans have a leg up here as both fans of the 56 and 79 versions are voting, and lets face it nothing is beating The Thing (and IMO nothing should).
I'll defend Signs - to a degree at least. It has some great high-tension set pieces, particularly when the alien is trapped in the pantry. But, yeah, the film still goes hell bound off the rails by the time it's over.
I put my vote in for Body Snatchers. For any version, although I haven't seen the Kidman one. I really just love the concept. No space ships. No laser guns. No army battles. They just take you in your sleep. Brilliant.
Battleship? Only joking. Lifeforce for me although I do love The Thing and Predator but never really thought of them as alien invasion movies.
Between The Thing or the remake of Invasion of the Body Snatchers this is pretty difficult but had to give it to Body Snatchers. The invasion is dealt with so perfectly and it shows the effect of the invasion on a metropolitan scale, and not just a group of guys working in the middle of nowhere. And it's much more subtle compared to The Thing Don't get me wrong though, The Thing is genius. I do love Signs and Dark Skies too.
I love The Thing and Night of the Creeps, but neither had the big dumb cheese staying power of Predator. When I was a kid and saw that for the first time, it blew my brains. Yes, it is nowhere near the caliber of several of these films, but damn if it isn't endlessly watchable and enjoyable. /Nostalgia
I keep thinking about this poll, and thinking there must be many missing from the list because of how diverse the options are. District 9 came to mind the other day as a glaring omission.