Ta Phrom is, in my opinion, the most evokative of all the Angkor Temples. The images of Ta Phrom are just as prominent as Angkor Wat and mostly centre around the massive trees seen growing all around the temple grounds and actually on the roofs of the temple itself. These trees are anywhere upto 500 years old now, around the time when the temples were abandoned. They have the consistency of sponge and therefore don't inflict any great damage on the structures.
This is the only major temple not to have been comprehensively restored and offers a glimpse of what confronted the French explorers all those years ago, when the Angkor complex was rediscovered. If you can only visit one temple, this is the one.