Second only to his "winning smile" powers.
Here's the problem: For the entirety of his film career, Loki forgets about his illusions the moment he's around someone who can kick his ass. Which is unfortunate, because with enough scheming and foresight -- you know, two traits he's supposed to have -- it's an all but unbeatable super power. Iron Man shoots a volley of rockets at Loki, but oh no! It wasn't actually Loki at all, but an innocent hot dog cart. Loki's really over there. Black Widow is on it: She combat leaps onto his face, only to realize seconds later that she's sexily wrapping her legs around a furious beehive. The point is that with half a brain, Loki could use his illusions to turn every battle against the Avengers into a series of hilarious Wile E. Coyote accidents.
To his credit, there was one brief moment in which Loki remembered he could conjure holograms, and it worked perfectly. Here is that brilliant move in its entirety:
"It's my brother, the master of illusions, suspiciously standing right in front of a prison cell! This is the perfect time for an all-or-nothing tackle!"
Loki could disguise himself as the president and sign an executive order for stricter bow and arrow control -- that takes care of like 15 percent of the Avengers right there. He could turn public opinion against the group by disguising himself as Jenny McCarthy and telling America's stupid that gamma rays cause autism. And if Hulk gets pissed off at that, at the very least, he could hide on the other side of the room instead of doing this: