I don’t think it was Captain Pike. It was Commodore Decker.
No question that any Vorlon or Shadow vessel could do it – again, over breakfast.
The Death Star seemed kinda limited to obliterating planets. The Empire was oddly dependent on ground assaults when it came to actual conquering, so I think we’d have a better chance then you think.
I don’t think the Death Star was LIMITED to planets. Just that the Empire was run by bastards. If they’d wanted to destroy a single continent – the equivalent of Australia – they could’ve.
My cite is information drawn directly from my ass.
It did have lower power blasts it could do fairly rapidly. I’m sure it wasn’t using planet-blaster mode when it was lighting up all those Mon-Calimari vessels in ROTJ.
And if worse came to worse, they could land and roll around crushing all that stand against them. 
Ennesby could conquer the planet all by his lonesome, as could probably any other single member of Tagon’s Toughs. We have no terraport defenses, remember. It might take a while, but what can you do against someone who can instantaneously appear anywhere on the planet’s surface, or anywhere else in the Galaxy, and who wields weapons powered by total conversion of any available mass including air?
That’s not so. Yes, the Earth is at the bottom of a gravity well, but still, it’s a very small target compared to the lunar orbit. If ‘Serenity’ is in a stable Earth orbit when she releases the rock, the rock will continue in a stable Earth orbit. If she’s in a hyberbolic orbit that would closely approach the Earth and then leave for parts unknown in the solar system, that would apply too.
On the other hand, if Serenity is approaching the Earth without angular momentum and passes the lunar orbit, then yes, you could release the rock, make a significant course change, and be reasonably sure that you’d get an impact. (It would be harder to be so sure about what point on the surface would be ground zero, as far as I can tell.)
Why are you supplying actual FACTS? I SAID my citation was ass-derived, damn it! 
Most Warhammer 40,000/Battlefleet Gothic capital ships could do it, by wiping out all life on the Earth’s surface. The Planet Killer takes it one step further, and is capable of destroying Earth itself.
Wiping out all life on Earth’s surface is not conquering the Earth. Conquering the Earth of necessity involves a lot of property damage, true, but you also have to enslave the men, stampede the women, and rape the cattle.
Plus, they have plenty of demonstration targets available, easily visible to our telescopes, within our solar system. A test shot at Mars, expanding the asteroid belt, would probably do wonders for our attitude.
See, that’s the part that ain’t gonna work out so well for ET. As long as they nuke us from orbit, where we can’t get to them, they can abuse us at will. If they come down to the surface with hostile intent, they’ll have to fight us. Maybe they’ll win, maybe not. But they will know they’ve been in a fight. And when they finally get me, I’ll be bringing the boys from Valhalla back with me for round 2…
Dora. In a millisecond. Hell, she wouldn’t have to fire a shot. She’d just sweet-talk Earth’s computers into giving up.
The Millenium Falcon – she may not look like much, but she’s got it where it counts.
The SDF-1 – Imagine performing a space folder over Washington. Of course, it’ll just work that one time and the SDF-1 will take a year to get back from wherever it folded from.
Peacekeeper Command Carrier – I have not doubt that it’s got enough firepower to turn much of Earth into molten slag.
TARDIS – I’m sure there’s a way it could counquer Earth.
Vogon star ship – Or just destroy Earth to make way for the hyperspace bypass.
That’s quite an internet-penis you have there. Tell me more about how you’ll destroy Space Marines with your rifle.
Of Culture vessels, certainly a General Offensive Unit, and probably General Contact Units, but conquest by force is really not the Culture’s style. More likely would be the landing of operatives from Special Circumstances to work political and social mischief.
Since nobody has bothered to mention the Andromeda Ascendant yet, it’s worth noting that she carried forty missile launchers capable of firing 8 rounds per second each. Each missile can carry up to a twenty megaton warhead. In one case it was mentioned that Andromeda could complete depopulate a densely inhabited world in under two minutes. I don’t see how we could begin to mount a defense against this. If more subtle methods are called for then planetary warfare bots with multiple weapon configurations are available.
The Sulaco could do it, they’ve got sharp sticks.
Actually, I gonna say that the real question is what warship with interstellar capabilites do we have a chance of defeating.
Gay Deceiver could maybe swing it, with some creative planning. Has the added bonus of really pissing off Fred Phelps.
Can anybody clarify how the half of these ships are going to CONQUER Earth, as opposed to just beating it up a bit?
Serenity? Millenium Falcon? Really? They have to land sometime, then they would be screwed!
The Death Star’s probably big enough that just parking it too close to the planet would have some fun gravitational effects.