Poison is the most resisted element, with a fairly large number of enemies being outright immune. Con is also a very common ability to be high, and is almost never low, meaning a large number of creatures have middling to good Con saves. IMX, it's also typically much easier to hit with a spell attack compared to a saving throw as levels increase. AC progression is relatively flat, but higher level creatures have many saving throw defenses like Magic Resistance as well as inflated Con for more hp. Furthermore, when I want to crush opponents with weak saves, I just use regular spells since those almost always do. Finally, it's also incredibly short range (10') making it compete with melee-only cantrips like Shocking Grasp and Shillelagh.
If I'm a Druid, I take Produce Flame, Shillelagh, and (probably) Thorn Whip first.
If I'm a Warlock, I take Eldritch Blast and Chill Touch first.
If I'm a Wizard or Sorcerer, I take Chill Touch, Fire Bolt, Ray of Frost, or Shocking Grasp first.
I'd still take Poison Spray before Create Bonfire, though. I need a concentration cantrip like another hole in the head.
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