The original trip to 1955...especially the first time I saw it. You see the speedometer go to 88 and you know the time circuits are on so you know time travel is going to happen but you don't know what EXACTLY what it's gonna be like from Marty's POV...and then, one second, outside the windshield, from Marty's POV, we see him about to crash into a Photo Booth at the mall...next second, FLASH and he's in an empty field about to crash into a scarecrow...a sudden vivid juxtaposition between present and past, modern and primitive, that is a great way to depict travelling back in time.
I also like the return from 1885 at the end of BTTF3...we see the Delorean disappear in 1885 the instant before it would have gone into Shonash Ravine. We then see the locomotive wreck and we see the ravine. Then, we see a shot of the same place 100 years later, an instant flashforward before our eyes. And then suddenly, out of nowhere, we hear three sonic booms and the Delorean materialises before our eyes, unchanged and still on the same path, even though everything around it has changed so much in the same place...and Marty is back home at last!