Tie breaker between Aizaik's Metal Augment, AquaticPanic's Mystical Mist, and Gravity Monkey's Steam Engine.
Item Name: Mystical Mist
Effect: If holder is a Volcanion, when switching in, sets Mist, Aqua Ring and Life Dew; cannot be knocked off when held by Volcanion.
Name: Metal Augment
Effect: When held by Volcanion, the user gains the Steel type, and the power of the user's STAB attacks is increased by 30%. Effects that grant Pokemon an additional type replace Volcanion's Steel type. This item cannot be removed from Volcanion.
Item Name: Steam Engine
Effect: If the holder is Volcanion, the first time it uses a Fire-type move, its attacking stat is multiplied by 2 while using a Water-type attack as long as it remains active and has this Ability.
Volcanion is currently C rank in OU for its Choice Specs set that is sometimes run on rain teams. None of these items outperform Choice Specs in rain if damage output is our metric -- Metal Augment doesn't grant as much of a damage buff, while Steam Engine would require using Flamethrower in rain before boosting to power levels where the rain isn't even needed, but setting up gives the opponent a free turn to switch in an offensive check. Therefore, these items should be evaluated for making Volcanion worth using outside of rain. Let's analyze each item:
Item Name: Mystical Mist
Effect: If holder is a Volcanion, when switching in, sets Mist, Aqua Ring and Life Dew; cannot be knocked off when held by Volcanion.
The closest comparison to this item is Heavy Duty Boots + Leftovers; Mist is a ribbon bonus that is of vague and little value on its own. Activating Life Dew compensates for the 25% Stealth Rock damage that should be assumed, while Aqua Ring grants the same benefits as Leftovers. This is situationally better than Leftovers + HDB -- if Stealth Rock is off of the field, that compensation becomes a watered-down regenerator. This item gives Volcanion defensive tools, which isn't an approach that I like on it's own -- as a grounded, stealth rock vulnerable, toxic-susceptible Pokemon without recovery moves, I fear that this item will only be enough to compensate for some of these disadvantages. If that is the case, Volcanion would end up being unimpressive both defensively and offensively, because you are giving up an offensive item in order to use it. Unless someone could show me some good evidence of this improving Volcanion's matchups in OU, this option isn't very appealing to me.
Item Name: Steam Engine
Effect: If the holder is Volcanion, the first time it uses a Fire-type move, its attacking stat is multiplied by 2 while using a Water-type attack as long as it remains active and has this Ability.
This option focuses on beefing up Volcanion's power as a pseudo Nasty Plot, as Gravity Monkey said. It only powers up Water type moves, but can be set up while attacking. Frankly, what I like about this item the least is its wording, which has two problems. Firstly, the wording 'the first time it uses a fire type move' implies that this only happens once, but the item isn't consumable, so I have a feeling that it is meant to reset every time that Volcanion switches in, which is the only way that this item is good. Secondly, the item also appears to reference itself as an "Ability", which is poorly phrased. Putting these problems aside and dealing with the intention instead of the execution, I would like to compare this item to Choice Specs. Firstly, Choice Specs is way better in the rain, but even outside of rain I don't think that the comparison is favorable for Steam Engine. Let's assume that Volcanion has 2 turns to act before it has to switch out or be knocked out -- which should be common, because Volcanion is a slow Pokemon that is Stealth Rock weak, so we can assume that it can be revenge killed by anything faster than it that can deal at least 75% damage to it. Choice Specs Volcanion is strong enough to 2HKO just about anything without Water Absorb, and Steam Engine boosted Volcanion will probably manage similar 1HKOs on turn 2. This comes with two disadvantages, however. Firstly, Volcanion actually does less damage in 2 turns with Steam Engine than with Choice Specs -- because you have to use the weaker Flamethrower or the less accurate Fire Blast for less damage than a Specs boosted Steam Eruption turn 1, doubling the strength of your Steam Eruption on turn 2 makes for less net damage than simply boosting your Steam Eruption right away by 1.5 times. And after either item picks up that KO, usually either Volcanion will have to switch out, be sacked, or the game will have already been decided. Thus, Steam Engine realistically never overtakes Specs. This comparison becomes even less flattering if Volcanion only gets 1 turn to attack -- which will happen, because some OU Pokemon can switch in on resisted hits and outspeed Volcanion to threaten it. In this case, Steam Engine grants Volcanion only 1 weak attack, while Specs grants Volcanion 1 strong attack. Specs is the better item. Even if you try to claim that Choice Lock is too much of a disadvantage, you have to remember that Steam Engine also hamstrings your moves -- to benefit from it, you have to use a fire-type move followed by a water-type move. Steam Engine doesn't improve Volcanion; it runs into problems that have made most slow Nasty Plot sets irrelevant.
Name: Metal Augment
Effect: When held by Volcanion, the user gains the Steel type, and the power of the user's STAB attacks is increased by 30%. Effects that grant Pokemon an additional type replace Volcanion's Steel type. This item cannot be removed from Volcanion.
This is my submission, and I wrote about why it is justified in my original submission post. I'll show why it is the best option here, though, by comparing it to Mystical Mist and Choice Specs. Gaining the Steel type makes Volcanion significantly more useful by giving it more opportunities to switch in. Being no longer Stealth Rock weak allows Volcanion to take greater advantage of its natural bulk, and Steel's resistances give Volcanion more opportunities to come in on resisted hits. While this effect isn't as pronounced as Mystical Mist's healing allows, Metal Augment's Life Orb boost doesn't give up Volcanions offensive might, allowing Volcanion to better take advantage of these switchin opportunities with offensive retaliation. Because Volcanion's defensive shortcomings are what hold Volcanion back from being good in OU, but Volcanion's offensive strengths are what have allowed Volcanion to hold onto a niche, it makes a lot more sense to me to use Metal Augment's targeted defensive and offensive boon than to have Mystical Myst re-spec Volcanion into a defensive build.
The comparison between Metal Augment and Choice Specs can benefit from looking at Life Orb. Volcanion doesn't bother with Life Orb -- it wears down Volcanion even more quickly and doesn't provide as much damage, so there is consensus that Specs is the better option. Now, however, Volcanion's longevity is
increased instead of decreased, Volcanion gains new switchin opportunities (which translates to more attacks!), Volcanion gains Steel STAB for additional coverage (admittedly, not the biggest perk), and Volcanion can switch moves freely to be less prediction reliant and possibly also run Defog. Choice Specs will definitely keep its place on Rain, but Metal Augment will do the best job at getting Volcanion into new team builds. I designed this item because I knew that Volcanion needed a better tailored submission, and I remain convicted that this is its best option.
My ranking of Volcanion's options are as follows: Metal Augment as the best option, Mystical Mist as a decent option, and Steam Engine as an unproductive option. Of course, as someone who submitted, I won't be voting, and I do have an interest in making the case for Metal Augment.
(Dracovish! Dracovish! Dracovish! Dracovish! Be revived and ruin OU!) 
AquaticPanic, TheUltraFinder, Gravity Monkey
Tapler
Tapler
Aizaik, AquaticPanic
Rocky Helmet
