This is a list of example forces for Killteam. They’re not all good, and certainly not all winners and at least one is there just because I find it funny and want to see somebody play it because it’ll be an interesting evening for all involved. I’m not saying which though. (lists to come later when I add them)

Adeptus Astartes, because you can’t trademark the name Space Marines

The boring “standard” army for 40k. Genetically modified soldiers who sometimes even remember that normal humans exist and matter. Available in all colours of the rainbow, including the Rainbow Warriors which were invented back when GW weren’t 100% grimdark. There’s no difference between the different chapters in this game, it’s just colours at the moment and not rules. New to 40k recently are Primaris Marines, which are a lot taller than old marines and never look like badgers, which the original 1987 marines did. You’ll want to play these if you’re into lists that are good at everything.
My current Dark Angels list
A better? list

Deathwatch

When the Imperial Inquisition need an army they select the best from all the marines and have their own army. They’re marines basically, but with more interesting equipment and a boring black paint job.

Grey Knights

Space marines that are even easier to paint because all you need is a silver spray can. They would be totally dull if it wasn’t for their scanners like ability to make their enemies heads explode just by thinking really hard. They’re reasonably tough and can ignore the advantages of an invulnerable save that make some of the other armies resilient. They’re also quite expensive points wise so you can get a squad of these together in real life in about 45 seconds including waiting for the glue and paint to dry. Not the most varied kill team, but the brain exploding makes them interesting.

Astra Militarum. Imperial Guard by a more trademark friendly name.

The grunts of the 40k universe. Think Aliens. Think Starship Troopers. Think “why is everything about them quite so Stalingrad?” Luckily for the guardsmen there are no Commissars in Killteam to shoot them randomly in order to improve morale for the rest of the squad and they actually have a rifle for every trooper.

Adeptus Mechanicus

One of the more recent armies. Machine worshipping engineers from Mars. They’re the ones who do the making of things for the Imperium, and they sure do love cogs and a nice bit of mahogany on their weapons. They’re actually quite interesting fluff wise despite how I make them sound and as a recent army their models are fantastic…

Heretic Astartes

Marines that went bad 10,000 years ago and now enjoy having spikey armour and a questionable relationship with demons. Probably just misunderstood.

Death Guard

Heretic marines who worship the god Nurgle. They are all about life in all its forms, unfortunately most of those forms tend to be disease and things living in rotten flesh. On the plus side they have Nurglings who are the coolest thing in 40k due to their fantastic set of poses on the recent releases of models. On the minus side you don’t get to use Nurglings in Killteam so it’s just slightly smelly marines and Poxwalkers, who are slightly gone off people. Expect lots of oozing and body parts falling off at awkward times. Quite hard to kill.

Thousand Sons

Heretic marines with a rather Egyptian look and armour full of dust instead of an actual person because you really should be more careful when you use magic in order to make your marines better. They share the Grey Knight’s enjoyment of making their enemies heads explode but take considerably longer to paint.

Asuryani. A space elf by any other name would be the Eldar.

“Classic” Eldar models that nobody pays attention to in proper 40k because they aren’t that good. They’re OK in Killteam though.

Drukhari. Used to be Dark Eldar

Eldar who like having fun, if by fun you mean so much debauchery it actually created a god. There’s only two unit types, the shooty ones and the stabby ones but they’re very good at shooting and very good at stabbing too.

Harlequins

Space Clowns with the most annoying paint jobs to actually paint that you can imagine. Very fast. Very survivable. Very deadly in close combat. For some reason they don’t know that guns that can fire further than 12” exist, but it hardly matters when you’re that fast.

Necons

Another “easy to paint” army. Imagine an army of skeletons, but actually they’re skeletons of the Terminator and just like Arnie they have a habit of getting up again when you think you’ve killed them.

Orks

Orks are da best. They’re also the only race in 40k who actually enjoy themselves because they just like a good fight. The race with the humour if you want something a bit lighter. Well slightly lighter as they’ll still rip your head off for fun. Not that great at shooting but have quite a few options.

T’Au

Space communist cow fish. A young race who are all optimistic and think that they can expand their empire and things are on the up. Their dictionary doesn’t have the word ‘Grimdark’. Good at shooting, might have knives or something for close combat maybe but you probably don’t want to be in a position to have to find out if you play them. They don’t get their big stompy mechs from proper 40k, which is a shame.

Tyranids

Think Alien and Starship Troopers. Bugs that just want to eat the universe. An unstoppable force who will undoubtably be the eventual winner in the 40k universe.

Genestealer Cults

Before the Tyranids turn up they send Genestealers who implant people with their young and what do you mean this sounds like a film you’ve seen? It’s actually one of the more interesting ideas in the 40k universe as you get people with various amounts of genestealer-ness living around humans and nobody quite realising what’s going on until it’s far too late and you’ve been eviscerated. They have a lot of repurposed mining equipment as guns.

Rogue Traders

In 40k a Rogue Trader is somebody who is actually allowed to go off and explore without being told off by the inquisition. They’re being added to the game in a few weeks so ignore them for now.

The Gellerpox Infected

Icky creatures created by Nurgle (see also Death Guard). A Geller field is a thing on a spaceship that stops the chaos gods from being able to get in when you use faster than light travel. Sound like the film Event Horizon to you? Well that was kind of written as a 40k film so just think that. Being added in a few weeks so ignore them for now.