Although Hitmonlee is relatively fast, it can still find itself outpaced by many Pokemon, and it also can easily be revenge killed by Choice Scarf users due to its low bulk. However, it can fix these problems with Unburden, which raises its Speed to 546 and allows it to outrun practically every Pokemon in the metagame, including all relevant Choice Scarf users. Be sure not to activate Unburden too early within the match, because there are usually many Pokemon that need to be eliminated before Hitmonlee can clean up, particularly Ghost-types, Psychic-types, and priority users.
Although Close Combat is the weaker of the two STABs Hitmonlee has available, it has perfect accuracy and is not lacking in power by any means, while the defense drops from the move are rather irrelevant because Hitmonlee won't get outsped and will get defeated by priority regardless because of its frailty. High Jump Kick offers a bit more power, but its 90% accuracy can ruin the chance of a late-game sweep. Stone Edge covers Flying-type Pokemon and also hits bulky Pokemon such as Rotom or Uxie harder than anything else Hitmonlee has. Although a priority move may seem useless with Unburden, it can still be useful for activating the ability. Mach Punch is preferred for this slot, as a Fighting Gem-boosted Mach Punch can finish off faster foes, and after Unburden activates, it can help Hitmonlee avoid getting hit by other priority moves. If you really need to activate Unburden, Fake Out with a Normal Gem guarantees the foe cannot strike unless it is a Ghost-type. However, be warned that this pretty much takes your item slot away, removing Hitmonlee's boosts on its main attacks. For the last filler slot, Return hits Psychic-types harder than any of Hitmonlee's other moves, while Earthquake hits Qwilfish hard, although neither do that much to their respective targets.
The item choice depends on the priority move. A Fighting Gem is generally more useful, because that way Mach Punch or the main STAB can activate Unburden. However, as mentioned, if Fake Out is used, a Normal Gem will ensure that the move activates Unburden. You could use other odd Gems to activate Unburden, such as a Ground Gem to power up Earthquake and hit targets such as Qwilfish for a ton of damage, and let Hitmonlee use its STAB moves early-game to try and avoid activating Unburden too early, but you must be sure that you will consistently be using the move that activates the Gem. Adamant is preferred because Hitmonlee will outspeed anything that isn't an unconventional Choice Scarf user—it outspeeds even Choice Scarf Cinccino.
Before Hitmonlee starts sweeping, Psychic- and Ghost-types need to be removed, as Hitmonlee cannot hit most of them very hard. Due to this, Absol, Spiritomb, and Drapion make good partners, as they have both Pursuit and alternative STAB options to take them down. There also exist many Poison-types in RU that do not take very much damage from Earthquake, such as Amoonguss and Qwilfish, that can stop Hitmonlee's sweep. Sigilyph can take down both of them or set up as they flee. Hitmonlee can also switch into Dark attacks that may be targeted at Psychic-types and has Mach Punch to bypass possible Sucker Punches. Finally, as almost all priority murders Hitmonlee if it cannot finish off the foe with Mach Punch, Alomomola makes a good partner, as it makes a great switch into physical attacks, most notably Entei, which packs Extreme Speed.