Any time of year is fine if you arrange your own transport round the island. All the cruise tours and day trips seem to visit the same sites in the same order, so just look at their itineraries and do it in reverse.
Back in November we had pretty much every site to ourselves apart from around midday. about 6 years ago
Easter Island is an amazing place and definetly worth the visit. If you can go at any time, then low season is from around May to October where there will be far fewer crowds and the Island will be nice and peaceful. If you have to go in summer, then do as Steve suggests and try and get to the sites either early in the morning or late in the afternoon before the cruise tour passengers arrive. about 6 years ago