Invasion of Astro Monster/Monster Zero for me.
I mean, it's the best movie up there for one, but also it's got the coolest alien invasion elements: coolest spaceships, coolest alien technology, coolest alien uniforms, coolest alien characters, most developed aliens in terms of plot detail.
The Killaks are kinda dull, but I really like the Nebula M Spacehunter and Black Hole aliens and all their low-rent '70s charms, too. I just dig the concept of aliens using monsters to conquer Earth, even if it is a totally unrealistic and reliant of cartoon logic. I was so happy when we found out that that was the ultimate pot of Pacific Rim (and hope those guys are developed further in the sequel).