well, lucarionite probably isn't underrated, but most stuff prefer the -ate stones, especially altarianite.
Altarianite actually is not great for most stuff, it's just incredible for some. It's got too much wasted in SpA for physical attackers, and too much in Atk for special attackers. It's only good if you're already really fast or, preferably, means of boosting speed, since really fast is relative here, or anyone w/ Espeed. It's incredible on those, but honestly, it dumps so much in both offensive stats that it's really not great, for the most part. It's incredible for a few specific mons.
Now, as a final addition to my last two incredibly long and time-consuming posts:
Sharpedonite would be awesome on Gyarados. It'd give it actual presence against offense, because of the speed. Oh wait, it doesn't give it dark type. No stab crunch. Maybe it wouldn't be that great for him in particular, but it'd be good on some other stuff. Weavile comes to mind. Lol, again. Speaking of which, Weavile could be really good w/ a lot of stuff. Kinda high-risk/high-reward, but totally worth it. it's probably the best sharpedonite user, but there's bound to be others.
I don't care what yall say about aggronite, especially on offense. I love it. It opens SO many opportunities to set up. Bulky dragon dancers could boost to absolutely silly levels. But don't listen to me. -.- notables: Dnite, double dance Lando-T, Feraligatr, Trevenant could make a decent tank, tornadus could be good- defensively it's a great typing, and it's got defiant pre-mega, plus decent 145 attack, idk, could be good. I'm gonna be using it on offense, no matter what all you heathens say.
Aerodactylite would be really good for a lot of decently fast, but not quite fast enough, physical threats. Metagrossite is similar to it, the only difference being what mon you're using, in choosing between the two stones. Metagrossite gives a lot of speed and a little attack, aero gives quite a lot of attack and some speed. So it just comes down to what you need more. Weavile, for example (I'm gonna keep coming back to that one) would almost undoubtedly prefer aerodactylite, while, say, scizor would probably rather metagrossite. They are both very good options for a lot of physical offensive threats. Honestly, it's not really worth the time for me to name possible user I can think of, there's a ton and they're pretty obvious. Zardite X is the same, but it adds dragon and lots of power and physical bulk, but no speed. So if you're already fast, that's the way to go.
Zardite Y/red orb is a similar case to the one above. "Would x mon rather more Def or SpD?" Also, red orb adds fire, which is good, but it can't support its team. These two are some of the best options I can think of for a lot of stuff though. The power of sun-boosted fireblast is crazy. Red orb also gives dragons the glorious fire/dragon typing of which legends have spoken. Salamence could be really w/ red orb; it would have ever so slightly more power than zard y, access to Draco meteor, and a lot better physical bulk for somewhat less special bulk. Oh yeah, and a lot better attack, too, which could be really good for lurring stuff. Actually, it's even better than its special attack. You could go either direction w/ it, a dancer or just a mixed/ special wallbreaker. As for dragon dance, it has fire fang which, while incredibly low base-powered, would be boosted by both sun and stab, and would be further boosted once you've dragon danced. However, it's outpowered by fireblast, even w/ 0 investment and an negative nature. I think it'd actually be surprisingly strong. 100 speed at +1 would outspeed every relevant scarfer and everything short of deo-d that I can think of. Except maybe some weird scarf mewtwo. But that's not the only option; volcarona would be a good candidate, Roserade, as I remember
The Pizza Man pointed out forever ago, is really cool, basically any ground types w/ good fire coverage would be good, etc.
Swampertite has some really good options, it gives a huge attack buff, good increase to bulk, and a little Spe and SpA. Dragonite stands out to me as the most obvious, it's got massive attack great speed w/ swift swim and a number of moves that work well w/ rain, like hurricane, thunder and waterfall, but any number of physically oriented water types could work very well.
Gardevoirite is good. Kinda self explanatory. Whatever you run is basically Gardevoir Mark II, so, not a whole lot to say here. Stuff w/o great fairy STAB would love it especially. Like togekiss.
Blue orb is naturally gonna be good, especially for physical waters like Gyarados. Obviously. Next.
Banettite would be great for offensive teams in a variety of roles. It'd work well on pangoro as someone said, can't remember who, I think it was
thdhted, to create momentum w/ parting short, or cheesy priority shuffling shenanigans, or just like sableye or thundy in OU. It's really just held back in standard because Banette is a trash base, but it gives a ton of attack and an incredible ability. Offense in particular would love the ability to set a sweeper back to 0 in order to get some momentum back, especially.
Sceptilite probs isn't the best, but it adds dragon, which is very good. This could be used to get rid of an unfavorable secondary types on dragons, or to add it to, in particular, fairies, steels, and waters. Also fire, but most would probably prefer something else. Jirachi could be very good w/ it, as well as togekiss, and kingdra, though it's already a dragon. Yup, that's about it. Fairies, as altaria has demonstrated, don't even need dragon stab, so lack of dragon coverage would probably not be a problem for them.
Blastoisinite is incredibly powerful on stuff that would appreciate it, esp. Hydreigon and luc, but provides no speed. This could be easily worked around tho, especially for Hydreigon. Lucs bulk is pretty bad though, so it may not be as good. But it's stabs would be strong.
Venusaurite could be good for bulky offense. It provides a decent increase to special attack and attack as well, along with ridiculous bulk. Thick fat is way good too. Notable stuff: celebi, lots of already tanky stuff, Dnite, Mamo maybe, yeah, you get the jist.
Houndoominite could be very good. Nothing in particular that I'm thinking of rn, but there's definitely quite a few things that'd like it. Ironically, it gives +40 Def.
Heracronite on SS cloyster could be really good. Not a lot gets much to utilize it w/, unfortunately. The speed drop is killer.
Galladite is good, but outclassed by lopunnite for the most part, probably. However, if you like the bulk, and want to keep your type unchanged, it'd be the better of the two.
Latiosite, while incredibly stupid, would be good for a lot of physical or mixed fire, electric, steel, and rock types. Victini would be really good. Luxray and Electivire might actually be good- cobalion and terrakion would be pretty good too.
Lastly, scizorite would be pretty good on a lot of things, breloom for one would like it a lot. It'd basically be exactly like the difference between base and mega scizor. The swords dance set in particular would benefit greatly from the increased bulk. Sadly, it's still slow. But there are plenty of others that'd really appreciate it, aegislash will be an animal w/ on the off chance it is actually unbanned, it'd be really good for pursuit trappers (like Weavile), lots of stuff could possibly use it w/ aerial ace for flying stab, idk what else, but technician is really good. Cloyster, drapion and scolipede could use it w/ twineedle for a 75 BP move w/ a 40% chance to poison, not to mention, cloyster would like the power of all its many multihit moves boosted by technician, including, and basically limited to, ice shard, rock blast, and icicle spear.
And I think that's all I got... Finally.