Okay, so. I was thinking, and... yeah, there aren't a ton of Kill Teams you can build in one box. It's unfortunate, and I decided to look and see what you COULD do in one box. So I went into Battlescribe and GeeDubs' website, and started working on ideas.
The parameters for a "one box Kill Team" are that you must be able to make a standard 100-point Kill Team with legal equipment options in a single purchase. Said single purchase must be a single-squad box, not a box set of any kind. The resulting Kill Team must have at least 3 Specialists (except in cases where three Specialists are impossible due to model cost) and must be in the 90-100 point range, a range I consider reasonable for 100-point Kill Teams. Options must be drawn from the core book and the Elites expansion (as well as that one White Dwarf about the Kroot), not Commander. The goal is to find a one box Kill Team for each playable race. We'll go in the order Battlescribe decided to arrange them in the roster from start to finish, through all playable races in Kill Team aside from the Rogue Trader races (Elucidian Starstriders and Gellerpox Infected) and Blackstone Fortress exclusives, and counting Primaris separately. I'll only be looking at one option for each race with the correct count or a surplus of models. Let's begin.
We'll start with Kroot, because... reasons. Kroot only have a couple options. You can have Carnivores, Krootox Riders and Hounds. Since the only option that can be a Leader is a Carnivore, you're stuck using Carnivores. ...Huh, Carnivores come in boxes of 16. 16 Kroot Carnivores in Kill Team is 96 points, and they can take like five different Specialisms, meaning... yeah, Kroot Carnivores are a one-box Kill Team, and a pretty okay one at that.
Seems Battlescribe decided to start being logical here, and gave me Spess Mehreens next. Termies are the obvious option - they're like 30 points apiece and come in a box of five, so you can totally do them, Space Hulk style - but they're tough to fit into a Kill Team, so I looked at Tacticals. And... yeah, you can do a good Kill Team with nothing but a Tactical box, and still have plenty of stuff left over. My recommendation is to do Dark Angels, combi-plasma and plasma pistol for the Sergeant, plasma gunner Specialist, missile launcher Specialist and Veteran Tactical specialist, with three basic Tacticals for filler. It's a full 100-point Kill Team.
And then we look at Primaris separately. I think it's important to note here that I said one BOX, not one UNIT. For the above Tactical KT, you can absolutely do one or two as Veterans of some kind if you wanna mix it up a little, and for this one, we'll be looking at the multi-part Infiltrator/Incursor kit. It's easy to get a variety of models here relative to other Primaris options since it's a two-build kit, and you have tons of stuff left over because you only need, like, half the kit to do it.
Mm, Custodes. They're insane for Kill Team. They have zero board control, but they outclass everything else in the game to the point where they often don't need it. Funny thing is, Allarus Termies actually AREN'T a one-box Kill Team... because they're too expensive. Clocking in at 67 points apiece, you can't even field two of them in a 100-point Kill Team. So get the Custodian Guard box, since you can field a 100-point of them.
AdMech are easy because they were one of the first factions in Kill Team; the original starter box had nothing but Genestealer Cults and Skitarii, after all. So I decided I'd look at a box which was NOT the regular Skitarii box that the Kill Team box set had. AdMech are good for the same reason as the Infiltrator box; they're all two-build kits, so you can have a mix. Problem is... the Ranger/Vanguard box is actually the only Kill Team-compatible AdMech box which IS a one-box Kill Team. So... yeah, that's unfortunate. Still, you get options, and I actually have a Kill Team from that box, so yeah.
Astra Militarum are... interesting, in that they don't have a one-box Kill Team outside of the Ogryn box. Can't do Tempestus in one box because they're a five-man box designed for a ten-man Kill Team, and fielding a full Kill Team of Guardsmen has the same problem; ten-man box for a twenty-man Kill Team. Ogryn, on the other hand... yeah, you can do them, and they're even pretty good for it because of their two-build nature, same as Infiltrators and AdMech. Granted, Bullgryn are way more expensive, but... still. Just build the kit as a Bullgryn Bone'ead and two Ogryn and you're ready to rock.
As far as I can tell, there actually aren't any Asuryani one-box Kill Teams. Wraithguard are promising, but you can't field one as a Leader. Striking Scorpions are promising because of their high cost and six models per box, but nope, can't do it. You can't do Banshees because, at present, they aren't out as a single box. Guardians are too cheap, even with ten models in a box. It's the unfortunate truth of Asuryani Kill Teams.
Ah, Daemons. Daemons only have one option for a one-box Kill Team: Horrors. You can take an Iridescent, an Icon Bearer, a Hornblower and seven Pink Horrors and it comes out to within our range. Bloodletters, Plaguebearers and Daemonettes can't do it, and there aren't any other Daemon options for Kill Team.
Death Guard are easy. Grab a Plague Marine box, done. You even get some spare parts left over. Next!
For Deathwatch, you actually have to do a Primaris Kill Team or take expensive stuff on the regular Deathwatch Kill Team kit. I recommend going piecemeal and thematic and running the actual Kill Team kit. After all, Deathwatch is the origin in the 40k universe of the term Kill Team.
Drukhari are aaaaalmost in the same boat as the Asuryani, if not for Wyches - but that's boring, because they were an actual one-box recommended GeeDubs-approved Kill Team for a while and got packed with terrain. But hey, it works, so...
Genestealer Cults were a starter faction for Kill Team alongside AdMech. I actually recommend the Aberrant kit, because it gives you chonky bois so you're smashing things like the Custodes almost, and it's easier to get up to points with a couple Aberrants than with, say, Acolytes or Neophytes - although the dual-build of the Acolytes is quite appealing to me.
Grey Knights are expensive and all their Kill Team options are one-box. Next!
Harleys... literally only have one option, and if you vary their equipment, yeah, it becomes a one-box Kill Team, but you have to take stuff more expensive than their basic equipment.
Chaos Space Marines. Tacticals but spiky. I have a headache, so I'm putting less effort into these explanations, and I apologize. Next!
With Necrons, the Triarch Praetorian/Lychguard box is the only one which can make a 100-point Kill Team. You can take five Lychguard with standard equipment and that's 100 points exactly. Yay for old Pariahs!
For Orks, it's easy: Nobz or Meganobz. Meganobz are like Custodes but not as good and you get less of them, and Nobz are pretty okay elite infantry.
I had high hopes for the T'au. I thought the Stealthsuit box would work. It, uh... didn't. So... Kroot...? I guess. Or Crisis Suits. Crisis Suits are allowed.
Thousand Sons. Rubrics. Easy. Assault cannon guy as a Heavy, maybe a Demolitions with a flamer.
Tyranids can only be done with Warriors. They're real easy to do in two boxes (Hormagaunts, Lictor) but you can ONLY do a one-box with Warriors.