It's so hard to pick, but I definitely love the mexicana complex kings the best. I think that aesthetically, the thayeri come out marginally in first for me. Mex mex are a very close second. I'd say that both are definitely underrated and beautiful animals!
Thayeri/Nuevo Leon/Variable kings are quite awesome in my book because, while they have virtually no 'morphs', they're amazingly polymorphic. If you google pictures of thayeri clutches you'll see the crazy variability in a single clutch. It's quite awesomeThey come in colors ranging from greys and tans to bright orange and yellow, with banding from narrow pinstripe to the 'milk snake' phase. My guy Mel is a yellow pinstripe leonis phase. I'm hoping his head and tail keep turning more orange (it's hard to see in this pic but his tail is more orange than his body)
Mex mex are beautiful as well and have some cool color/pattern phases...check out the ultra granite, I want one of those!