This is surprisingly useful.

The cantrip gust could conceivably disperse this 3rd level spell in 1 round.

Hear me out, what if the gas ignites or explodes when it's VERY close to fire. That would be wonderful!

There are very rare cases where it would be remotely conceivable for a cantrip to counter a 3rd level spell. This is not one of them. 

Quote from MadPuppy57 >>

The cantrip gust could conceivably disperse this 3rd level spell in 1 round.

 

Gust cantrip? I would rule 4 rounds/moderate wind. It's just a small blast that doesn't push with enough force to cause damage. 

Gust of Wind (2nd level spell) would disperse it in 1 round. 

No, the Gust cantrip cannot disperse a 20' radius sphere. The most it might do is move the spells effect 5' away from them.

What if the circle of the spores druid could learn this spell? Would that make logical sense?

Another spell that would've been fun on an alchemist~

A good/bad spell depending on a LOT of factors.

The good

1. Those inside the cloud are heavily obscured from their respective enemies same as fog like spells/environment 
   1. This means those heavily obscured can't be target of spells 
   2. Enemies have disadvantage to attack if you have a familiar present, acting as your eyes for enemies outside of the cloud. You have advantage on attack rolls against these targets.
   3. Enemies have disadvantage to attack if using a bat familiar for enemies inside and outside the cloud so long as they are within the blindsight "vision" of the bat familiar. You have advantage on attack rolls to these targets 

*references for the above points can be found in [PHB] under Chapter 8: Environment - Vision and light.

what I don't like about this spell:

1. DM can interpret the rules as creatures can avoid having to make a saving throw just by holding heir breaths before running into the cloud (you can hold your breath equal to 1 minute + 1 extra minute per con mod [PHB] - Chapter 8: The Environment - Suffocating 
2. It doesn't have any real incentive for enemies NOT to go in melee.
   1. Mob runs in and assuming they bump into you - they can make an attack (straight roll unless you have blindsight) and then risk an opportunity attack to leave the cloud (depends on whether there is enough movement left). Attacks of opportunity are not that significant by casters unless they have the warcaster feat and even then it's uses a reaction from that could be used for a counterspell. 

What changes I would like to see :

• Add an errata that specifically state that creatures that have special breathing mechanic (Tortles, Warforged etc) are not affected by the stinking cloud. This allows only a select percentage of creatures to benefit from this spell
• For those affected by the spell - add a save roll for each time a creature runs into the cloud for the first time for each round or if they start at the beginning of their turn in the stinking cloud.

Unfortunately, as written the spell is more bad than good, although not entirely for the reasons you point out. 

• First, there is nothing in the spells description that says that creatures which must breathe and aren't immune to the poisoned condition can hold their breath, so assume that this is the purpose of the saving throw.  To do otherwise would make it an even worse spell than it already is.
• Second, although I agree that a creature should need to make a save the first time on a turn that it moves into the cloud, the way the spell is written it doesn't really mean anything if that turn isn't their own.  This is because failing the save only denies them their action, and the only time a creature can use its action is on its own turn.
• Finally, don't forget that you must be able to see a creature leaving your reach in order to be able to make an attack of opportunity against it.  So, in that sense you could (as the spell is written) enter the cloud to make an attack and then withdraw without provoking an attack of opportunity, provided the target lacks blindsight. 

I think the best way to make this spell on par as a 3rd level spell is (as you suggest) to require a saving throw the first time a creature enters the cloud on a turn or if it starts its turn within the cloud.  A failed save should mean the creature becomes incapacitated (can't take actions or reactions) until the start of its next turn.  It still wouldn't affect creatures immune to the poisoned condition, of which there are many, but at least it would be worth something against those that weren't.  As it is now, a creature can just dash through the cloud without suffering any ill effects at all, move into it and still take their action or bonus action, or move out of it and lose no more than a single action (if they fail their save). 

Quote from MadPuppy57 >>

The cantrip gust could conceivably disperse this 3rd level spell in 1 round.

 

 I mean, as long as the caster's not helplessly retching at the time. Casters tend to have great Con saves, right?

Seriously if you make a utility play like gust and have the presence of mind to use it in the perfect emergency, you deserve to succeed.

Quote from MadPuppy57 >>

The cantrip gust could conceivably disperse this 3rd level spell in 1 round.

 

 No, but the second-level spell Gust of Wind could do it in 4.

Quote from dakingsman>>

 Second, although I agree that a creature should need to make a save the first time on a turn that it moves into the cloud, the way the spell is written it doesn't really mean anything if that turn isn't their own.  This is because failing the save only denies them their action, and the only time a creature can use its action is on its own turn.

Actually I think it makes sense that a creature has to be in the cloud to be affected; if you see a cloud of noxious gas hanging in place, you would absolutely hold your breath if you need to run through it. If you're in it though, then you've got prolonged exposure, and it's probably not just breathing it that's bad; if it's like tear gas then getting it in the eys and on the skin for any length of time is also bad.

I think the best way to make this spell on par as a 3rd level spell is (as you suggest) to require a saving throw the first time a creature enters the cloud on a turn or if it starts its turn within the cloud.  A failed save should mean the creature becomes incapacitated (can't take actions or reactions) until the start of its next turn.  It still wouldn't affect creatures immune to the poisoned condition, of which there are many, but at least it would be worth something against those that weren't.  As it is now, a creature can just dash through the cloud without suffering any ill effects at all, move into it and still take their action or bonus action, or move out of it and lose no more than a single action (if they fail their save). 

The creatures effectively are incapacitated, because if you can't use your action you get no bonus action, and the most you can do is move out of the cloud while "wretching and reeling". It's essentially a spell to rob multiple enemies of an entire turn.

I like it because as with Darkness and Fog Cloud it creates an instant line of sight blocking obstruction, but one that can also rob enemies of their turn. Unlike Darkness/Fog Cloud where enemies can simply walk out of it unscathed and then act normally, if you fail the save you still lose that entire turn. Plus with some basic pushing shenanigans from an Open Hand Monk, the Crusher feat and similar, you can force enemies to start their turn within it for multiple rounds, potentially rendering enemies with lower CON completely harmless.

In fact the best way to think of the spell is as a levelled up version of Darkness, which itself is a levelled version of Fog Cloud. If your goal is only to incapacitate then there are better options, like Entangle or Hypnotic Pattern, but it's the double whammy of incapacitate and blocking line of sight that makes Stinking Cloud useful. Plus it's just a really funny spell conceptually. 😝

It has good synergy with zombies. Have an invisible necromancer delay it until the end of the round casting it as a reaction after all the zombies rush in and begin to grapple.  Breaking free of a grapple requires an action and cannot be attempted if the con save fails.  When party members escape the cloud the necromancer and skeletons can fight the free party members outside the cloud while the grappling zombies in the stink cloud continue to split the party.