Located in the tropical region of Retalhuleu, in the south west of the country, Takalik Abaj is a unique Mayan archaeological site in Guatemala. It's unique in that it has an Olmec legacy (and the mark of a long Olmec presence before the Mayans arrived) and, having existed for 3,500 years, is the oldest site in the country... Mayan ceremonies are also still held there, making it a very lively site.
I personally really enjoyed the guided tour. It was very interesting and I experienced it under the very hot climate of what the Guatemalans call "the coast" (the tropical coastal region and the inland region).
The site and its monuments are rather small in size. It's important to remember in fact that, on the one hand, all of the country's archaeological sites have been abandoned for centuries either prior to or following the Spanish invasion; and, on the other hand, many were deliberately destroyed by invaders during wars and colonisation. Nothing in Takalik Abaj even vaguely comes close to the headstones in Quiriguá, the temples of Tikal and Palenque, or the excellent preservation of the headstones in Copán, for example, which is why the site is rarely included in trips to Guatemala.
I'd recommend visiting this site especially if you're exploring the Retalhuleu region, or even if you're following a road along the Pacific coast. It's great for archaeology enthusiasts who might have read about it, or for people who are lucky enough to be accompanied, as I was, by a good guide.
Note: the site has been included on the advisory UNESCO World Heritage list since 2002, and its inclusion is something of a precedence. In September 2014, the Deputy Minister of Cultural and Natural Heritage went to Paris to present the progress of the case. This will be revealed in June 2015, at the annual update of World Heritage, if Takalik Abaj becomes the fourth Guatemalan UNESCO site.