This is an ingenious moveset spread across the ages, constructed piece by piece with painstaking care in the arching caverns and ancient machinery of the most devious and cunning hive mind in the history of anything.
This moveset brings Unown's fullest potential into the light. Its most powerful move, Hidden Power, rips through Magikarp, Hoppip, and Feebas with its unimaginable power, 2HKOing and OHKOing Magikarp and Hoppip with Hidden Power Ice, respectively. A neutral Hidden Power gives Feebas the ol' one-two-hit KO. When facing such opponents, Unown is unbeatable.
As if this wasn't proof enough of Unown's superiority, Hidden Power Ice OHKOs Flygon. Transporting to prehistoric ages, Unown can 2HKO the powerful T-rex, Tyranitar, with Hidden Power Fighting. And going even higher up on the evolutionary ladder, Groudon can be 2HKOed by Hidden Power Ice.
Team Options
Supporting Unown is essential, for it performs best in an environment it is comfortable with. As Unown can sometimes get temperamental when dropped into a situation outside of what it is used to (caves, incest), it is wise to pair Unown with familiar environmental hazards. Stealth Rock can be set in play by Donphan and Chansey. Dual screens are also helpful, as they remind Unown of the old, mysterious days, steeling him against attacks. Uxie, Mesprit, and Gardevoir are excellent choices.
A Dugtrio is a popular choice for befriending your Unown. They often reminisce about caving, and if Dugtrio is feeling particularly adventurous, Unown can explore Dugtrio’s lower half. In return, Dugtrio traps Chansey, who otherwise walls Unown, and 2HKOs her with Earthquake.
Having a Normal-type on your team also helps; as Unown has been secluded for thousands of years, it needs a normal Pokémon to teach it the ropes in today’s modern world. Chansey is a magnificent choice; it is immune to Ghost-type moves such as Shadow Ball that threaten Unown. Chansey also supplies paralysis support, which raises Unown’s low self-esteem about its Speed.
Fighting-types like Hitmontop and Hitmonlee will help toughen Unown up; they will also take Dark-type moves for it. Scarf Primeape can come in and scare Dark-type Pokémon out with Close Combat, but he can't take multiple hits well.
Being an outcast, Unown is often a target for Pursuit. To help Unown feel less insecure and lonely, it is suggested to pair it with a teammate that allows you to get a free Swords Dance, Calm Mind, or Nasty Plot in, while at the same time not take too much from a switch-less Pursuit. Superb choices are Lucario, Suicune, and Infernape.
Taking care of Unown is difficult, but ultimately worthwhile. With the right care and attention, it will grow into a wonderful Pokémon. Good luck in your endeavor—we wish you the best!
Other Options
It can only use a differently typed Hidden Power, for its power is secret, hidden, and locked away.
Opinion
The first time I saw Unown, they were hovering in a line above the floor, staring at me with malicious cyclops-eyes. Upon further inspection, I discovered their general shape to be Latin letters. They spelled:
THE MOTHER OF CHAOS’ MOTHER HAVING SEX WITH THREE MANS! OH NO!
I fled from the cave, eyes bleeding from unsolicited dirty images. I was so blinded by terror I tripped over a rock, fell, and scraped my arms. And when I hauled myself up, there they were. Standing seductively in front of three men was chaos’ grandmother.
Counters
Anything that resists its Hidden Power or anything that has good Special Defense beats the mighty Unown. Chansey walls Unown, even as a wielder of Hidden Power Fighting. Grumpig is also a good candidate to defeat Unown.