One of my personal favorites is the red-lined ribbon snake, Thamnopsis proximus rubrilineatus. It's a small snake that likes to be near water--creeks, ponds, etc. This picture is from our lily pond:
Another Texas beauty is the speckled racer (doesn't live near me now--saw it as a kid once); it's a bigger snake with dark scales that have a golden center to each one. Sometimes they look bluish or greenish because of a blue base under the black. This link is to a "field herp" site with several pictures: http://tinyurl.com/ps5y7by
The western coachwhip (which we do have on the place) is even longer, but hard to photograph because it's so long and so very fast. Here's a picture of the head of one. Here the scales look whitish on the edges, but in many lights they seem to be lit from within--they're the same taupe as on top of hte heat, but the edges "glos (look at the scales of the snout--that little "glow" is what you see on all the scales of the snake in good light. Also note the round iris in both the red-lined ribbon snake and the coachwhip--they're non-venomous snakes and in the US, that's a reliable sign you don't have a pit viper. The coachwhips are elegant and very fast, including climbing in brush.
It's a shiny bright green, shading from a little darker on top to more yellow--then the yellow on the lower side, and white belly scales.
We "should" have coral snakes on our land--we have the right habitat for the local kind--but I haven't seen one. So here's a link to a lot more about these beautifully colored snakes than I knew before: http://www.reptilesmagazine.com/Snakes/Wild-Snakes/Coral-Snakes/
My snake pictures (which include most of our identified snakes, including a big black-and-gold Texas rat snake) are scattered amongst all the other nature pictures and I should collect them all in one file. But I haven't done that yet.
Non-native to US beautiful snakes include (are not limited to) the reticulated python (http://www.reptilediscovery.com/retic.html), king cobra (http://www.arkive.org/king-cobra/ophiophagus-hannah/)--note that the pupils of its eyes are round, not slitted--it's only here that round pupils are a guarantee of non-venomous. And because there are too many beautiful snakes and it's time to fix dinner, here's a link for several at once: http://scribol.com/environment/10-most-beautiful-snakes-on-earth/12
I really don't think the western diamondback qualifies in the "beautiful snake" contest. But again: "eye of the beholder."