40k, without a doubt.
the reasons? scale and philosophy.
using the imperium as an example; the latter is easy to explain. imperial philosophy dictates complete prosecution against a foe in truly total terms untill the foe is defeated; if it cant win on the ground, and it cant win with its fleets, all it needs is a few hours to make whatever its fighting over a barren rock, completely bereft of economic or strategic use for all of eternity. nothing within star trek cannon has anything resembling the resolve to approach this level of total warfare, and thus it would lose before the opening shot.

in the case of the former; one segmentum of the imperium of man, obscurus for example, is larger than the size of all known space as shown in tng era canon. said segmentum controls untold thousands of capital level warships which dwarf anything i know of the the star trek canon by an order of magnitude. segmentum ground forces (imperial guard and planetary defence forces) are so inpconprehensably vast that not even the imperium is able to guess at just how powerful its military is.

the weakest ships of the imperiums navy used in actual fleet engagements, cobra class destroyers, outgun sovereign class ships heavily and the cobra’s gellar field, the forcefield equivalent, can sustain bombardment from ships of about equal power with a reasonable chance of survival. the would probably, though im not entirely sure, be able to outrange federations ships, which means they can dictate range. the majority of a star-trek starships power comes from the various directed energy weapons, afaik non of them can be used at warp speeds, which severely hampers a starships ability to use tactical warps to its advantage.

imperial battalions (the smallest self contained and self-supporting units) number in the thousands in terms of line troopers, not including support. application of mass, as well as the fact that the lasgun (standard imperial weaponry as well as the basic field weapon, literally the weakest used by front line units) is a true directed energy weapon, and therefor problematic for ST defences, dictates that imperial forces would win any and every ground engagement.

in terms of overall strategic manueverability…im not sure in truth. warp travel is faster than sublight travel by default, and is (pretty) safe, which nominally gives ST races an advantage in speed. it is also exceptionally slow compared to imperial FTL travel. assuming the warp is calm an imperial battlefleet could cross the entire alpha quadrant in a matter of a day or so, where it takes months for a federation starship to do the same. afaik only borg transwarp can even approach this level of speed.

in technological terms, no race in star trek has a chance outside of ftl travel. their defences are, by an indeterminate number of orders of magnitude weaker than they would need to be to survive the opening salvo of an imperial warship, whilst, with the acception of 8472 of the canon races, their ability to provide massed firepower again pales in comparison. the entire federation fleet (some thousands of ships i think by the dominion wars?) could unload all of its weaponry continuously against an imperial ship of heavy cruiser class or greater untill their warp drives were exhausted, their capacitors cracked from age, and every torpedo in the alpha quadrant expended, and the imperial warship would then still be able to wipe them all out in a matter of hours; keep in mind the largest starships outside of borg cubes in star trek-romulan warbirds and sovereign class ships are perhaps the size of an imperial corvette.

in terms of industrial base of potential energy output, again, the ST races literally have a snow balls chance in hell. single forgeworlds in 40k are able to produce enough military equipment to maintain whole imperial armies, which number in the millions of line troopers, with tens of thousands of support vehicles, artillery support and air support, not to mention titans…which are preposterous in scope. a single forge world such as armageddon could literally outproduce the entirety of the star trek galaxy without going into some extreme level of output, using just its standard trade links to maintain said production.

furthermore; we have the sheer scale of difference in terms of endurance. the federation, klingons, romulans etc…are able to prosecute wars for years at a time, with losses of hundreds or thousands of their ships representing near crippling losses, and these factions are exhausted utterly by the end of these wars. the imperial war machine is able to fight on all fronts at all times, simultaneasouly fighting defensive wars alongside succesful wars of conquest, using only a fraction of its full potential military power (once again, the imperium is so monolithically powerful it doesnt even know just how much military power it has) and has done for over 10,000 years. it is nowhere near exhaustion, and far from catastrophic defeat. the imperium can lose military forces that would leave the federation irreversably depopulated without even noticing, let alone flinching.

after all this we have the fact that the imperium is slowly losing its eternal war against the other powers of the 40k galaxy, and it is nevertheless so powerful that a fraction of its raw power (not including gods, trans-dimensional beings or 1-letter superbeings) is more than equal to everything the ST canon could throw at it in any pitched battle.

i honestly dont even get how there can be serious arguments on this matter. as powerful the star trek factions are, and as cool as i think they are, the imperium alone is so much more powerful that it is not a matter of how many weeks the ST canon races could last in war with the federation before they cease to exist in political terms, its a question of how many hours they could do so. games workshop have literally invented something which is impossible for any other universe to deal with.

@some dude
Nintendo wtfpwned Warhammer dont lie