Galvantula will not be of much OU use, I think, for five reasons:
1. It is rather weak, with a shallow base 97 SpAtk. Even in UU, it is outclassed offensively by Zapdos and Heliolisk.
2. It is very, very frail, and having a Rocks Weakness doesn't add to its viability. Having a unique typing is of no great use to it either.
3. It is fast enough, tyong woth Keldeo and Terrakion, but misses out on the now-crucial base 110 speed tier, and unfortunately does not have other stats high enough to make up for that, even with LO Compoundeyes Thunder being a neat gimmick.
4. Its main niche on Sticky Web is rather a poor strategy in OU, with Bisharp and Defog everywhere. Even so, Shuckle makes a more reliable user of that move, and can hopefully get up SR too.
5. The combination of all the above provide literally no reason to use Galvantula over Thundurus, Mega Manectric, Raikou, Volcarona, or even Yanmega. It will not merit much usage in OU outside of a niche gimmick, I think.
As for UU mons that are worthy of OU use, aside from the obvious ones like Gyarados, Chesnaught, Mamoswine, Alakazam, etc. that you have listed, I can say the following are pretty underrated threats:
1. Yanmega: Modest Choice Specs Tinted Lens Air Slash can 2HKO Sdef Skarmory and offensive Heatran. This is not a joke. This is power. A set with Bug Buzz/Air Slash/HP Ice/U-turn can be potent as a wallbreaker and as a scout or offensive pivot.
2. Crawdaunt: Death by Claw. This thing will brutally murderize anything that dares to switch in or commits the simple crime of being slower than it. Allowing it SD is usually also death, unless you can outspeed its priority (looking at you, skill-less Talonflame...)
3. Azelf: Suicide leads ahoy! SR, Taunt, some attacking move, BOOM. No explanation needed, HO (especially birdspam) appreciates this.
3. Mega Sharpedo: Omnomnomnomnomnom.
4. Mega Beedrill: Pair this up with Lord Magnezone and you have one of the most effective and fun-to-use VoltTurn cores in OU. Beedrill is a terror, too, especially if it has Knock Off to remove shed shells. DragMag will appreciate this as well.
Other things that I mention but can't be bothered to type about: Mega Ampharos, Infernape, Mienshao, Cobalion, Shaymin and Doublade.