I'm trying to figure out which WH40K race/faction would do best in the Doctor Who universe, and I think the Orks for a number of reasons. As numerous as they are, they're too disorganized to seriously threaten the really powerful DW civilizations like the Time Lords or the galaxy-destroying human empire from Nightmare in Silver, and will therefore avoid pissing them off and getting wiped out/erased from existence as retribution (and if some random Ork empire does go and try to take out either of those too, it'll just be that one Ork empire that gets destroyed, or they might simply ignore it). They likely couldn't be subverted by villainous characters/races like the New Dalek Paradigm, the Cybermen or the Master due to the psychic Waaagh! field and the release of spores (well, the Nightmare in Silver Cybermen might be able to overpower/subvert the Waaagh! when converting individual Orks, but the constant release of spores could still potentially cause problems). One Ork can potentially rebuild everything from scratch. I mean, drop one Ork on a planet and give it time without it being harassed, and it'll release spores which will create more Orks, including Mekboyz, and they can start building colonies, vehicles, nastier weapons, spaceships, etc. No other individual member of a 40K race can do something like that (unless you count really high-end individuals like Primarchs or the God-Emperor, but I was simply imagining a basic Ork there that could be the one surviving refugee of a lost war with the Daleks/Cybermen/Sontarans/Hoothi/Rutans/humans). They wouldn't overwhelm everyone in the setting by any means, but honestly, they'd do better than the Imperium would.
What of Chaos and Necrons. Necrons because (don't they have their own time altering devices) plus they are a horrifying rape train of deadly weapons, and chaos because, well, it's chaos. Everything would have trouble dealing with chaos.
Oh, don't get me wrong, the Chaos Gods would cause a huge amount of damage and, based on the quotes about them slowly consuming the entire universe if the God-Emperor fails, they'd be an absolute nightmare to deal with for most of the lower-end factions like the New Dalek Paradigm or the Cybermen. However, due to the huge amounts of carnage and cosmic unrest they'd cause, eventually someone higher-up on the Whoniverse food chain like the Time Lords, the Eternals, the Chronovores or a Great Old One (note: DW Great Old Ones are much more powerful than the Cthulhu Mythos beings they're based on, they're actually much closer to Lovecraft's Outer Gods than his Old Ones) would notice them and get rid of them for all the damage they'd do. To give you an idea of how good the Time Lords are at dealing with reality-warping beings, each individual member of the Charon species could create a universe, on their own, and the Time Lords murdered all of them because they felt they weren't playing nice with the lesser races. There's also the Eye of Harmony, which the Time Lords used to erase magic from existence, and therefore all races dependent on magic, from existence, even though it was a fundamental part of the universe itself, such as gravity. So, at the very least, they could use it to do what the C'tan planned to do and cut off the Warp from the material universe. As for the Necrons, they'd do a better job of avoiding the big boys' attention than Chaos would (especially if they have C'tan with them, namely competent C'tan like the Deceiver who know that flying around waving their dicks would be a bad idea), but I think the Orks would do just a smidgen better at surviving because of how they can rebuild everything from just one Ork, although I suppose the Necrons could do a similar thing to what the Cybermen do when they're running low on numbers and go into hibernation (hell, I could actually see some Necron factions teaming up with the Cybermen). By the way, how good is Necron time travel? To give you an idea of how good Whoniverse time travel is, one Sontaran with nothing by a damaged scout ship can travel hundreds of years into the future - and the other temporal powers in DW consider the Sontaran Empire as a whole to be a complete joke. And right here is the biggest reason the Orks would do so well in the Whoniverse; they'd be the best comedic relief that the show has had in fucking years.
The Astartes should just find a really powerful human faction like the NiS human empire to team up with and get mass-produced by them.
