DOTM by far, the music that played as we descended down to Cybertron was fantastic, and watching the ship weave in and out of those "pillars" or whatever they were in 3-D looked amazing. I still remember when I saw ROTF in cinemas and the first thing I thought was "why the fuck is there a Tiger in Africa???"
TF 1 for the emotion, DotM for the action. I think the first one wins out for me though. Between the intro and the Autobot arrival, that movie had a lot of nostalgic emotional high points. I was just so happy to be in a theatre and hear Peter Cullen speak as Optimus Prime - it was a rare feeling.
While the DOTM intro was very impressive, the follow up scene is almost as important. And DOTM dropped the ball hard. TF lead into Blackout raiding the base. Great. ROTF lead into Nest forces and Ironhide rolling up in Shanghai. Awesome. DOTM lead into Carly's ass filling the screen. And a domestic conversation with Sam. Wow. Way to kill the excitement right there Bay.
I won't necessarily agree that it "killed the excitement" (entirely), but I will concede that DotM was the odd one one for the formula of "awesome intro/backstory then jump into awesome action sequence then show some calmer human scene". DotM just basically switched the last two. I often wonder what the flow would've been like had it gone Intro-->Chernobyl-->Sam/Job. But then again, the intro with Cybertron/NASA/Sentinel for DotM basically made up for this trivial thing.
I just about lost it when I first heard Cullen's voice on the big screen waaay back in 07. I'd been waiting to see it for so long, and I couldn't believe I was finally getting to see my favorite characters come to life on the big screen. ROTF's opening was decent, but it just didn't have the kind of attention-grabbing feeling to it as I think it wanted to have. Prime saying that "our worlds have met before" just didn't feel as epic as it should have. Plus it could have set the stage for the Fallen better. And then DOTM came along. I wasn't disillusioned by ROTF or anything like that, but I was nowhere near expecting the sheer awesomeness that was getting to see THE Great War in live action. My goodness, that was easily one of the best moments in the entire trilogy. I'd have to go with this one for the best intro.