At 18" tall I would be inclined to stick with the more terrestrial snakes. I really like Drymarchon. There are plenty of Cribos available so you don't need to worry about the legalites of them in your state. Of course the more common cribos don't get as big as an Indigo, which you may not prefer. Yellow Tails get huge but I think they are pretty hard to find.
I used to keep several Coachwhips, mostly westerns. Mine were never aggressive. I don't know if I got lucky or what. I really enjoyed keeping those. I used to love having bigtime breeders of more common stuff come over and hand them 6' of beautiful reddish-pink Coachwhip. Most of them did not even know what is was. They always left impressed.
My favorite pet colubrid, and by that I mean completely placid, calm, and handleable was a Florida Kingsnake. I had one that was a monster. I understand Black Milks are very much the same.
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Current snakes:
0.0.1 Gonyosoma oxycephala - Java locale (green)
2.2 Gonyosoma janseni - Seleyar locale (all black)