devilmons

Triton a good choice (nat 5 so no alternative + cd reductions on 2/3), Tyron iffy choice (if he's a CC, he needs to max 2/3 or they're pointless and he tends to have his freeze ready for the next screen unskilled in ToA hard, as a DPS skillups don't do as much as you'd hope). You are almost positively going to need to keep Tyron as a CC, not a DPS, and I don't think it's worth the investment. I'm not saying there aren't times it'll help, and if I could directly spend devilmons on only his 3rd, I'd max it, but I can't. Mine is still unskilled and I'm in the ToA hard 70s (atm, we'll see how my patience goes clearing stages, it gets fussy). Baretta is more important to max, people tend to stop using Tyron very high up in ToA Hard but Baretta is used the whole way, and losing a turn off his ATB/dots is huge.

Ifrits are not very important to max besides Vero. Amir takes WAY too many and he's fussy to use, though he's usable. It's just such a stupidly large investment. I'm leveling mine as an experiment soon, I need some AEer to tag with my Dragon for AE bombing, and I don't have many choices. I don't want to speed-lock my Lushen to my Dragon, I made Charlotte (who also doesn't hit very hard) a vampire farmer, so I had few choices. Also, if built as a ganker, he can't use his ae stun the same way as Vero, he won't be fast, he won't be on violent, and he won't have good accuracy. If you build him to be fast and accurate, then he'll be Vero without the cleanse.

Theo won't have enough accuracy to dot in PVE reliably, you build him for pure spd/dps, don't have acc stats leftover (so anything he defbreaks or dots is gravy), which also affects his defbreaking whether you boost it to 70%/hit or leave it at 50. Unfortunately, despite being incredibly strong, damage skillups are NOT as good as they look on paper. If you gain 25% total on skillups to their first skills, they do not hit 25% harder. People tested it at maybe half the listed values for real-world benefits. I'm still tempted, because he's so good, but... I have 14 devilmons sitting on my storage and I'm on the fence using them on Theomars, despite using him constantly. Partially because I feel like he's going to get nerfed, partially because he really functions well without anything. Most people agree it isn't worth it unless you have nothing else and don't expect anything else. As for Tesarion, he actually will have some accuracy, and he can do okay damage if he sneaks in some crit/cd subs while staying speedy/tanky and violent, similar to how Vero can do some respectable damage. The question is simply how tanky you need him to be, because when you bring him to turn off scary passives, you're... inherently bringing him to teams with scary passives, and some hate him (e.g. critmillas, who will always pick him without outside interference). The accuracy and his tendency to be built speedy does mean he'll get more out of his skillups, but his dot rate isn't going to matter much in PVP, mostly whether he's helping break (and it depends whether you need him to break or just oblivion). He's certainly important, but devilmonning essentially a PVP-only monster is a bad idea when you don't have everything short of ToA hard on farm.

The best, by far, by far, by far, use of devilmons is cooldown reductions. At least, in terms of relevant skills. But it also matters how they're used. For example, pretend Chloe did not have nat-3 skillups. If you only used her as part of zerging (like Chloe/Lushen/Lushen/buffer, or Chloe/defbreak/dps/dps, or Chloe/defbreak/buff/meganuke), she needs no CD reductions. She goes in, she buffs, fight is won or terribly lost. If you use her in sustained fights, then being able to heal and buff more often is very very useful, and would be worth the pain of suffering through skillups to her 1st skill, which has a million ranks.

Then there's the matter of whether things do damage, and why Verdehile is never a choice, ever. He is very rarely used as a DPS (and isn't good at it, it's just a case of occasionally a comp might as well do it, mostly in PVP), so his numbers will be low no matter what, and unless you're using him in PVP, you basically never want him using his second skill. He gives more ATB from his first skill. The AI isn't smart, it will use the 2nd skill often if it's available. The second skill has a CD reduction, so you make him worse when used by the AI if you max his 2nd. An unskilled Verdehile is directly superior to a maxed one unless manual-controlled, and the only manual use of him is in PVP (on offense), and ToA (when doing stages hard enough you can't auto anymore). Manual use, of course maxed is technically better, because if you need to, the 2nd is on-demand more often, but realistically it doesn't do anything unless he's built for pure damage as part of very, very specific comps. The much more common spd/crit/hp builds wouldn't notice a difference from all the damage skillups.

Kaz (and all samurai, for that matter) are not worth devilmons, nowhere close. I use mine when I need a second water DPS for some reason (GW assigning two teams, or have room because of a poorly designed AD, which is rare in c3+ teams anyway), but a lot of ADs are shifting anti-water now because of all the Theomars running around. It only goes so far, because the heavier you go into wind, the easier it is to double Lushen. He's perfectly capable as a ganker, but skilling up takes WAY too many devilmons to be worth it. Would it make him better, sure, but he needs a lot, and a GW-available water monster is superior in almost every situation. He does have the luxury of a decent amount of +dmg plus CD reductions, but he has a lot of skill 1 ranks (and has a 2 turn CD on his 2nd when maxed, so he won't use it much), and the total of 16 skillups is just silly. I wasn't available enough to max him the Samurai weekend and even though I'm very close to his CD reductions, I just haven't. He's usable, but I would never devilmon him. My use of him was entirely predicated on the availability of the free skills from the HoH, and I chose him over the fire/wind because water's the most lacking. Fire, for example, has the Kahli and Jean options for pure ST burst, Wind has an absurd amount of great DPS, water's DPS is narrow. He's focused on pure ST, which is what you often want when not AE zerging in PVP, so he was an easy pick. You could spend 10 devilmons on him, have them spread around all his skills, and not notice his performance compared to unskilled. If they all went into his 2nd/3rd, then you'd notice. But then Theomars will take his place.

I should also mention that spending devilmons immediately is a bad idea unless something is definite. For example, spending them immediately on a Veromos is absolutely fine. He's used almost everywhere, he has the accuracy for both his 1/2 to matter, and becomes an entirely different monster with them. But if you're spreading them around, most will be basically wasted. I mentioned Tyron earlier. If he's not a dps, skillups that don't go into CD reductions do nothing. So if you spend a few, never reach a CD reduction, and then priorities shift, you've basically lost all those devilmons until you finish maxing him.

You also have to be careful spending devilmon on 4 stars that may get a HoH. We don't know if they'll make monsters fast enough to keep up with HoHs or not. If they do, then any monster that already had one is safe. If they don't, or there's a special event, there better be at least one other useful monster of that type. I'm glad I devilmonned Lushen, I've used him for maybe 8 months already, they STILL didn't make a Joker HoH, and Jojo is worth maxing, as well as a second Lushen if I pull one, so there's basically no way that's wasted. Jojo alone needs 7 + however many get wasted into his 1st, and can take up to 12. Chasun's more difficult. She's amazing, but they have to do her HoH eventually, and the water/fire are both... mediocre at best. So while they'd be worth getting HoH skillups for just in case, I would never want to devilmon them, so if I had a Chasun right now, I'd be nervous about devilmonning it. The only real use of 2 Chasuns is a spare in case one dies in a GW.