The Khmer, in my opinion, are probably one of the best Cultures in the game. They are extremely amazing, with a solid LT, powerful EU, and incredible EQ. 

The Legacy Trait is probably the weakest part of the kit (although that doesn't say much, the bar is really high). Compared to other Legacy Traits, it likely won't give you nearly as many yields as, say, the Ummayyads' Learned Friends or the Teuton's Spurred by Faith. However, getting +2 Industry on Works is still useful, and it can rather easily give you several dozen extra Industry. It's a pretty solid bonus. It's not one of the top Medieval Legacy Traits, but the rest of the Khmer's bonuses more than makes up for that. 

The Emblematic Unit is absolutely amazing. As with all elephants, the Dhanvī-gaja is one of the top Emblematic Units of the entire Era. It has higher Combat Strength than everything in the Era bar the Varangian Guards - and I'd argue that the Dhanvī-gaja is stronger, because of the fact that it's ranged and therefore does not take retaliation damage. You can even compare the Dhanvī-gaja to Arquebusiers from the next Era, and find that it's even stronger. Dhanvī-gaja are not only cheaper, they also have more Combat Strength, have more Movement, and can Move and Fire. Dhanvī-gaja arguably have better Strategic Resource requirements (2 Copper and 2 Iron vs 1 Iron and 1 Saltpetre), and they even have lower Upkeep! Overall, the Khmer would be more than capable of grabbing some land off their neighbors, simply because of how powerful the Dhanvī-gaja are.

But, even though the Dhanvī-gaja is extremely powerful, I believe that it's the Emblematic Quarter that's the highlight of the Khmer. The Baray is an absolutely amazing District, providing both Food and Industry. I feel like it's the epitome of the "tall" EQs, which I believe is fairly fitting for the Khmer. Both Food and Industry are probably the two most important yields that you need to build a tall City, which the Baray provides in spades. And of course, its abilities fit right into what tall playstyles want to do anyways - building lots of Districts and growing Population. Its two major abilities - Food from Districts and Industry from Population - synergize amazingly with each other. As you build more Districts, you get Food to grow Population, which gets you more Industry to build more Districts, and on and on. 

In a tall game in which you have multiple Territories attached to your Cities, the Baray can quite handily solve all of your Food problems on its own. The substantial amount of Food you'll get from simply building a District means that you don't ever need to build a Farmers Quarter - because after all, every District you build is basically a Farmers Quarter thanks to the extra Food that it'll provide to your Barays! It's within the realm of possibility for the Baray alone to bring your Cities to the 1 Population per Turn threshold, even if you don't take any Agrarian Cultures. 

Even when you're not going tall and attaching everything, the Baray is still an amazing District. It can quite easily provide a few dozen or so Food and Industry, which is no small feat.