Ahu Tongariki is a "do not miss" site of Rapa Nui. Because of that, I'm sure it gets crowded with tourists but we visited around noon in early April and were almost the only people there. It's an absolutely stunning site and very easy to set up a camera on a timer to take a shot of yourself or family that appears as if nobody else is there.
The only disappointment is that I wish there were garbage cans at some of the sites (or at least bags distributed so that you could pick up trash and take it back to your hotel). In particular, there is trash that washes up on the rocks behind Ahu Tongariki and I would have liked to do my part to clean that up a bit.