Do you specifically want a North American milk, or do you just want something flashy, or do you want something easy to handle, or what??
I am not ANY kind of milk expert, but I'll tell you what I happen to like. In North American milks, I am always impressed when I see pics of syspila. Very beautiful snakes. For showy milks in general, I really like Hondurans. Good size, blinding colors, easy to maintain, and fairly good to handle (they tend to be squiggly but not aggressive). But it all depends on what characteristics are important to you!
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0.1 Peruvian rainbow boa (Amaru)
2.0 Brazilian rainbow boas (Arco, Olho)
1.3.1 Honduran milksnakes (Chicchan, Chanir, Hari, TBA)
1.0 Thayeri kingsnake (Coatl)
0.0.1 Mexican black kingsnake (Mora)
2.7 corns (Cetto, Tolosa, Uce, TBA)
1,000,000.1,000,000 other critters