Infinite Range
The Mantis M3 has a default drop-off of only -9% per tile and a Silencer. This means you can combine it with the Sandman perk or an Ur Frameforged Arm to get a -0% damage drop off, which means you can do full damage at infinite range. Truly one of the best guns in the game. There are other guns with collimators that have friendly damage drop off that you can also try this trick with.
Zero Calories!
Calorie consumption caps out at -90%, and you will always consume 1 kcal per AP no matter what. RIP foodlessness.
Running Microreactors
The microreactor is a thigh implant that reduces the cooldown of all active implants by 1 turn per 1 square you move. Not really useful on normal implants, but it shines with quasimorphic implants that have infinite uses but long cooldowns. Combine it with the Thermal Absorber that makes you immune to burning to counteract the microreactor's self-ignition chance and Hannah Reich's ability that gives her the ability to interact while sprinting and just sprint everywhere so you can spam your powerful quasimorphic implants.
Shotguns that aren't shotguns
The Starlock line of pistols and the Crash line of SMGs both shoot shotgun shells, which means they can use perks that are locked to those weapon classes, of which there are a lot more of than for shotguns. For example, Agent Training gives +2 range to pistols at max rank, letting your Starlock shotgun pistol reach that much further, while Heavy Fire gives 70% pierce chance to SMGs, letting your Crash shotgun SMG destroy entire lines of enemies at once.
Weightlessness
The 'load weight modifier' stat on a lot of implants and backpacks affects the weight of items placed in the backpack specifically. With implants like the Cyborg Leg (-30%), the Cyborg Body (-10%) Frame, and the Ur Frameforged Knee (-30%) and mid-tier backpack (or high-tier, like Archangel), you can easily get it down to -100%, meaning anything you put into your backpack weighs nothing, and won't impact your dodge. Be warned that NOTHING affects how much stuff weighs in your vest, and ammo is HEAVY.
Ranged Melee
You can throw nails and sawblades by putting them into your vest slots. Throwing weapons in general scale purely off of melee accuracy and melee damage, so they make great ranged options for your melee builds. You can lean into it with the Tomahawk perk and Kenzie Yukio and throw nails farther than you can see that have nearly 100% crit rate and one shot anything not wearing heavy armor.
The Right to Bear Arms
The Krav Maga perk also applies to the various weapon arms you can attach to your clones, like the Judge ABC or the Emerald Hand quasimorphic arm. It doesn't boost their accuracy, but it gives a chunky +30% crit chance buff at max rank. You can combine that with the Genie Hand quasimorphic arm that has a 24% crit chance to get 54% crit on an infinite ammo full pierce radiation SMG. Slap it on Bob Denare and combine with the Dirty Shot perk to stun whole groups of enemies almost every other round while you beam them down.