This isn't necessarily from least to greatest. These are just one's I could think of off the top of my head.
1) Hearing Leonard Nimoy as Sentinel Prime for the first time. It was so surreal hearing Spock's voice coming out of a giant robot, in his likeness, no less. At same time, I was so in awe; it was like being in the presence of a god. His voice really fit the character of a seasoned Cybertronian warrior and definitely commanded respect.
2) Bumblebee crashing, throwing out Sam, transforming to bot mode and grabbing Sam in mid air, then transforming while we see Sam get put inside from his point of view!!!
3) Ironhide getting killed by Sentinel Prime. I'll admit, I had read by accident that he was going to bite the dust, but his death still left me shocked. And I was even more surprised that Sentinel was the one to do it. Also, I kind of liked Ironhide, so it made me a little sad.
4) Seeing all those people just get disintegrated by all the Decepticons firing on them was pretty shocking and disturbing to see. This movie definitely lives up to its title.
5) The entire wingsuit sequence. Man, my heart was racing the entire time because watching that whole scene in 3D made it feel like I was actually there!
6) After seeing Q get gunned down and then the Decepticon about to gun down Bumblebee, I was thinking, "No, you can't do it! You can't!" I really thought he was going to get killed off. In my opinion, out of all the things in these films, Sam and Bumblebee's relationship has always been the heart of these films. I think I was on the verge of tears.
Fortunately, Optimus came to save the day, so no biggie.
7) Optimus getting his arm cut off. Damn, just...damn.