You’re kidding right?, I doubt the Culture builds escape pods that couldn’t conquer the earth.
Assuming your definition of conquest includes forcing a surrender, a better question would be “what fictional spacecraft couldn’t conquer the Earth?”. Just being able to freely move around on the upside of the gravity well is a pretty unbeatable advantage at our current stage of development.
If you are talking about actual boots on the ground, infrastructure mostly intact conquering, you are limited to ships that can carry, construct, or subvert enough troops to do the job.
So you are talking about anything even slightly weaponised from the Culture (you could take the planet easily with a single effector, even the life pod probably has some pretty nifty nanotech constructors), maybe Star destroyer or up from the empire, the Enterprise D assuming they make actual effective use of their technology (so probably no :p), a bunch of stuff from EVE, and anything else really large or sufficiently high tech.
At the very least it has the capability to move the entire planet.
The Nostalgia For Infinity fromRevelation Space. With the ancient cache weapons they could pretty much conquer or destroy anything they wanted.
And if the novels are in any way canon, could materialise around the entire planet and the state of temporal grace within the TARDIS would prevent anything other than hand to hand combat.
Also - if you go back in time half a million years and land on the right bug or something, that’ll change history in just the right way so that in the 21st century humanity recognizes you as the rightful overlord without you having to do anything else to conquer them, right? 
A TNG photon torp had a max yield of 64 megatons, and that’s assuming 100% efficiency. That’s not that much bigger than real nukes. If they were in orbit (low orbit I imagine might be necessary though) and we launched hundreds or thousands of nukes at the same time, that should be able to destroy them. The problem is that they could dodge them or apply countermeasures of some sort, so we’d have to trick them somehow into letting their guard down.
Most of these suggestions would be fine for destroying a city or two, but then what happens? Say the Enterprise D turns up, blows up Washington, and the world leaders decide to surrender. They simply don’t have the manpower to hold on to the whole planet. A resistance movement would spring up overnight, and the next time they decide to transport something up from the planet, they discover it was a nuke, and its all over.
On the other hand, a Stardestroyer is pretty much designed to conquer and hold a planet. According to Wookiepedia, your basic Imperial class stardestroyer has just short of 50,000 crew on board, compared with Enterprise’s limited 1,000.
The only other ship I can think of designed for this is the one from Independence day. Turn up, kill the majority, and then have enough ground troops to actually invade. I’m thinking the mothership, but the individual saucers could probably do it too, it would take longer.
Hey, save it for the Number Six dance later on will ya?
Forgot about the a borg cube. Conceivably, you could conquer a planet with a single borg.
An Ur-Quan ship could probably do the job.
They’d never need to come down into low earth orbit. There were plenty of instances in the various Ster Trek shows of vessels using their weapons from high orbit with pin point accuracy. More to the point you think our missiles can reach into orbit? They’re not designed for that so I don’t know how you think they’re going to achieve that.
We don’t have hundreds of thousands of nukes. Since nuclear disarmament took effect and the US and Russia started decommissioning their stocks the entire world has less than 30,000 live missiles right now (and that’s a generous assessment). Cite and cite.
None of which are configured to hit a moving target. In orbit. With shields. And phasers.
In fairness to the Empire - everyone is dependent on ground assaults for conquering. If all you want is to blow things up, fine - you can do that from the air, or from space. But conquest - an actual, long-term takeover of your enemy’s government at gunpoint - requires boots on the ground. Even a terrifying ultimatum - “Do as we say or we’ll nuke you” - isn’t really sufficient. Even if the government complies, what of hard-liners among the population?
We have a good real-world example of this, actually: Japan. Nuking Hiroshima and Nagasaki almost certainly triggered Japanese surrender (please, let’s keep the debate about whether it would have happened eventually in another thread) - but the Allies didn’t just rely on the threat of nuclear holocaust to maintain control of that country. They put boots on the ground - it’s the only way to see what’s going on in real-time, adapt to changing circumstances flexibly, and (hopefully) win hearts and minds.
This. Nukes are unpleasant things to be on the wrong side of. Barring truly magical tech, they tend to ruin your whole day - and we’ve got a lot of them. We don’t have particularly impressive delivery systems - but if the bad guys are walking around on Earth, we don’t need them.
I think most any of the advanced races from StarGate could do it. The Go’auld motherships were pretty impressive and regualrly carried crews enough to conquer entire worlds or take them back from other Go’auld. And the Asgard could hand several of those ships their ass with a singel ship of theirs --of course, the Asgard simply didn’t have the numbers to maintain a takeover.
The Wraith were certainly capable of it, as they were shielded from nukes and their little stinger ships just scooped up people and took them back for feeding. The only problem withthe Warith is that they would have no desire to conquer, just to use us as a feeding ground.
The replicators…don’t even get me started on the replicators…
The Ashen proved they could do it by using bioweaponry which slowly eliminated the existing population by rendering them sterile (they had done it on other advanced worlds similar to ours).
Both the Orii and the Ancients could do it easily as well.
Oh, now you’re being silly. Gay has exactly one offensive weapon, a single laser cannon of surely limited utility; Zeb had self-defense in mind when he installed it, and his philosophy is cowardice whenever possible. And I don’t see what good the magic bathrooms are gonna do in a fight.
If Gay Deceiver can conquer the Earth, Nightcrawler can.
Though I began the thread thinking that Star Wars tech would have it all over Trek tech, this makes me think otherwise. A ruthless captain and company on any Enterprise but Archer’s could do a whole lot of damage without ever firing their phasers or torpedoes. Their sensors can detect our nukes whereever they are, and I’ll bet their computers can overcome ours too. Beam a small away team into a missile installation, arm the nukes, and pop out. Repeat a couple dozen times.
It won’t be an afternoon’s work, but in a month they could entirely strip our defenses.
Yeah, but they can establish space superiority and wreck our infrastructure and ability to sense (say by destroying all satellites and phasering all radar towers on the planet), fighting them devolves into a guerrilla war. Depending on the Scifi universe we might have a chance or simply be shit out of luck. If the Roger Young and Rico’s Roughnecks drop down, good luck taking out a unit with that level of maneuverability and ability to deploy tactical nukes.
Having a space based enemy that can hover in orbit and destroy cities at will would tamp down the resistance a bit. And a bunch of yahoos shouting “Wolverines!” with hunting rifles is cute and all, but ultimately Sci Fi wins because they don’t have to work under the constraints of reality.
If they want to actually hold and rule the Earth I’d imagine they’d break us down and then offer aid, food and power. Start up education camps for youngsters and build a loyal base of 21st century humans.
Which would work of course, until the protagonist has his girlfriend kidnapped by a local overseer and begins his Path to Revenge™.
For that matter they can scan and beam up any world leader they want. If Odo is on the ship (go Defiant) they can institute whatever changes that leader is capable of. Or they could put a bomb in his head, or poison his daughter with a virus that needs daily inoculations, or just make them disappear. Do that for a few weeks and watch the humans squirm.