So many to choose from, but I'll list a few to start with, so as to not hog all of the choices.
The Moody Blues - Out and In Mike Pinder was pretty much the king of the mellotron, as he is the guy who brought it to the forefront in rock music and did so many innovative things that were eventually replicated, so it is hard to pick just one moment of his, but Out and In is pretty much a mellotron orgasm feast from start to finish.
King Crimson - Starless The beginning of the song is incredible, of course, but the final minute of this song is just bananas, in large part thank to the mellotron.
Steven Wilson - The Watchmaker The mellotron in the middle section, underneath the flute solo, is so badass, and of course we cannot forget the ending.
Neal Morse - The Conclusion When it comes out of that little mini-break at around 7:40ish, the way the mellotron lays on top of the rest of music is nothing short of astonishing.
I'll let someone list what many will say is the best mellotron prog moment of the 70s...