Netlisting is a tabletop gamer term referring to a player's choices in a wargame where only the "best" options are taken, resulting in a difficult to beat list. It's not a term of endearment. Such lists are intentionally unbelievably broken, and require counter-netlisting (or luck/a large amount of skill) to beat.
The term comes from gamers who, rather than figure their armies out for themselves or build around a theme, simply find the best possible list on the internet.
Netlists have a tendency to stigmatize against playing certain armies, as they are pre-judged based on the worst they can be in some cases. Quite a bit of the bile expressed towards some factions largely stems from this, then woe be to you if your favorite models get buffed into the clouds in an update as the community starts howling about blatant favouritism shown towards your chosen faction.
On the other hand, players expecting the netlist who netlist themselves against it may find themselves dressed for the wrong fight if they face an entirely different force (although then beating the opposite player with your "sub-optimal" options may just result in an accusation of army-wide overpowering).
All but the most underpowered factions in a given game are capable of netlisting. For those who aren't, it's just the only way they can possibly win. Or so they say anyway...
Expect ample skub.
- Dwarfs: Dwarfs are notorious for their cheap and effective artillery and their decent gunlines. While Dwarf melee lists are fairly weak, few players look forward to facing Napoleon McBlownapart as cannons are ridiculously accurate, have ludicrous range, and blow away anything they want to kill with high damage volleys. This has made large models (some of which are among the most amazing Games Workshop has ever excreted, or are popular named characters) virtually always inferior to more rabble (reversing the "redshirts die first" trend handily). Couple this with a gunline and you have an army which sits in deployment and pewpews at you until (and if) you get into melee range, at which point it's just a mopup one way or the other.
- Empire: Same as Dwarfs, but with the addition of melee redshirts, fast heavy cavalry, and and fucking huge steam tanks. Plus cheap level 4 wizards with access to all core lores.
- High Elves: High Elves are an army that has rarely NOT had an overpowered option of at least one kind. In the current edition, this is the Banner of the World Dragon (renders unit near immune to magic damage) with White Lions of Chrace (Elves who are killy as fuck, on par with followers of Khorne) and Alarielle the Radiant with Lore of Life (powerful wizard, melts demons in proximity, renders her bunker of troops fearless, increases their killing potential, all on top of having the best possible lore). In the past, Teclis was notable (level 5 wizard who could not miscast).
- Vampire Counts/Undead Legion: Red Fury is an upgrade for vampires that allows them to make additional attacks as they kill enemies. Given that they are the best combat characters in the game, this makes them extremely overpowered. Terrorgheists are giant bat monsters that emit a scream which melts rank and file troops like butter while being fairly effective at all other tasks, so naturally most Vampire Counts players try to cram as many as possible into a list. Zombies are cheap and weak, but soak up damage like almost nothing else and can increase in a huge number beyond what they start with resulting in them being a mainstay to tarpit enemies.
- Lizardmen: the infamous double slann list was a staple of 7th edition lizards, but got nerfed to uselessness with 8th rules on rolling for the winds of magic. Skinks are actually a neverending source of rage, however, since fielding hordes of cheap poisoned weapon throwing skirmishers running everywhere is fucking annoying to deal with for everyone.
- Daemons of Chaos: Thanks to your pal "Ward Save", Daemons were THE strongest army of 7th edition, bar none. Able to dominate magic, fuck with opponent stats at will, and otherwise every single option had a ridiculous buff combo. Although Daemons largely ate the nerf bat in 8e, modern netlists are usually composed of mostly Nurgle models supplemented either by Slaaneshi or Tzeentchian wizards, and marbe a few Khornate or Slaanesh options to hit flanks with.
- Orcs & Goblins: One word; Fanatics. Batshit insane goblins high on shrooms swinging GIANT metal balls. You don't know what enemy unit has Fanatics until you get into combat with them. A populat list of yesteryear was nothing but generic goblin chaff and Fanatics which was technically unbeatable. It's no longer as good, but still useful and ripe for trolling others with.
- Eldar: Eldar have always historically been borderline OP, and certainly there are many lists they can run that will utterly annihilate any opposing force. This came to new heights in Codex: Eldar 6th Ed. and Codex: Craftworlds 7th Ed. The former spawned the now-infamous Serpent Shield bullshit, a truly very overpowered tactic that nobody could really counter and was really pretty unavoidable even for casual Eldar players (since the Wave Serpent was their only Dedicated Transport option). This was nerfed for the most part in Craftworlds, allowing the Serpent Shield to only be fired once per game and not coming back afterward, but this "compensated" for by greatly buffing the already-powerful Wraith units (with ranged Strength D weapons on <200pts elites choices and making the Evangelion a Garagantuan Creature) as well as dropping the points cost of everything in the codex, including Wraiths and Serpents. Put the former inside the latter for a guaranteed tableflip by turn 3.
- Tau: The standard nonsense you see from Tau hasn't changed from their inception: Battlesuit Spam. Whether it's your normal XV8s taken as Troops through Farsight Enclaves, or the sheer unstoppable bullshit that is the Riptide variant spamlist, the Weeaboo Space Communists are nasty as ever. With all the guns you can use (out of range? What's that? Some human thing?) and a Buffmander who can give his team all sorts of goodies, they'll be the biggest pains in your ass.
- In 8E, XV8's can't be taken as troops and are overcosted for what they do and commanders are limited to 1 per detachment. Instead of fixing things, a new type of spam was born, behold the golden age of the Fire Warrior, with Ethereals to give them FNP, markerlights to reroll 1s, negate cover, and give +1bs, Fireblades to give them an extra shot (3 shots per model in Rapid Fire range), drones for extra range or to fuck enemy charges, and Chapter tactics that let them overwatch on 5s and 6s or even add an additional 6 inches of range, you might even yearn for the days of suit spam, since at least they looked cool. (Riptides are still very viable)
- Necrons: Originally heralded as somewhat balanced, if powerful at launch, the edition update turned them into overpowered table destroyers whose sheer ridiculous, overpowered nonsense led to many fa/tg/uys to cry out in sorrow and immediately be silenced by RAGE. These days they're not quite the nonsensical win buttons they once were, but their real power lies in the extremely good Decurion formation and its sub-formations, which curb-stomped tournaments shortly after it came out and has quite literally rebuilt the way armies are constructed in 40K.
- Blood Angels: Openly game-breaking at launch, these assholes gradually got ratcheted back in subsequent errata. By the time of the edition update they weren't so bad anymore, and are rather mediocre out of all the power-armoured choices currently available.
- Grey Knights: Absolutely, completely, utterly table-destroying until the edition update. They have since been hit with the nerf bat hard, due both to the changes to their codex in 6th Ed and the AP2 proliferation that has built up in the game, making Terminator-based armies not nearly as viable as they once were.
Netlisting against other players is difficult, while many factions have inferior matchups in the form of a Lord leading the army there's few that are outright cheesy in a match. But against the AI in the campaign its another story. Most Netlists are either designed to automatically win Autoresolves by gaming how the computer thinks a match would go, or simply to cheese them on the battlefield using their predictable (or in some cases unpredictably idiotic) way of using the options they have.
- Empire Free Company Militia autoresolves at a ridiculous degree of strength compared to what they actually do in a battle.
- Anything with turrets and decent maneuverability and/or durability. Arc Dodging is the practice of weaving in and out of asteroids and larger ships while till shooting the piss out of anything. Combo with Twin Laser Turrets for maximum cheese. Particular offenders are the Outrider, Millennium Falcon, and TIE Aggressor.
- Spamming high attack options. TIE Phantom is the most notorious culprit.
- Emperor Palpatine. Just Emperor Palpatine.
- Dengar and his wife Manaroo fromed a powerful combo known only as Dengaroo. To clarify, Dengar became a 3-die turret which: attacked twice, never lost his focu s token, rerolled his own dice, rerolled your dice, and could even u-turn with 8000 stress. Mathematically Dengar was worth several lists-worth of points in effeciency, smiting Palpatine from the top lists. His reign forced his ship to be beaten to death and unplayability with the nerf bat.
Goultard and Nox are particularly powerful options, and most models have some form of combo with them. While most other lists are evenly matched, G/N lists are something to sweat facing.