I'll do a detailed review of my favorite, Balam, and make a few quick notes on the rest. Since they're all superior CaS, I really have to magnify the slightest faults to find anything to say. They're also good for similar reasons (aesthetically pleasing, creative use of special pieces, historically accurate, etc.) so I focus on what I feel can be improved. Consult the latest post on my gallery for my evaluation guidelines.
1. Balam (detailed review)
Aesthetics - the armor, cloth, and skin are evenly and tastefully distributed throughout the whole outfit while the varying colors (red helmet, green shoulders, gold armor, black skin), patterns (leopard, scales, floral, and geometric), and stickers (face tats, sun, and phoenix) make sure it doesn't look too much like a jumpsuit.
Creativity - Chinese helm + Roman ornament = Mayan goodness, and I don't even know what you did with the face tats.
Coherence - Midnight black skin is highly appropriate for dead, creepy civilizations (good artistic license, even if they were actually more brown than black). Flashy gold outfits, geometric prints, and animal prints typical of ancient South Americans. Flat shoulder plates are appropriately simple/crude for the time period.
2. Tupacan - Alotta subtle cultural context put into head, as you mentioned. I like Balam more though, mostly because he's just more obvious. Hell yeah, green shoes ftw. The iris ornament doesn't connect to the head as well as I would like though.
3. Quezacoatl - Headdress is both innovative and beautiful. Torso and lower body isn't quite as good. Priestesses usually wear more clothes. Fur trim gloves are too modern looking. Translucent fabrics on Elysium's footwear also anachronistic. Again, rainbow pattern doesn't look quite as natural on the feathers.
4. Machupichu - Appropriately super-sized for a king. Epic crown, as usual. Tiger lily armlets too fancy for the time period.