If modded games count:
My favorite empire was probably the Soviet Union I created in RFC Dawn of Civilization on Marathon speed. It started when some guy had posted a savegame in the 1700s of a completely broke and backwards Russia with some weird civics (Republic? Capitalism? Get out of here!) for the situation. Also that guy had automated literally everything. I don't mean just workers or what tiles cities work, even what cities build! The only thing they manually controlled was military units. The land was barely improved, and there were forests everywhere! Strangely enough while Moscow was surrounded by unimproved riverside forests the Deer near St Petesburg had no forest.
I assume that's because of a random event. Anyway, I literally had to start building Wealth in all cities and put the gold slider to 100% just to avoid going broke on the second turn of taking over. Fortunately the previous player had almost finished the Winter Palace (national wonder that works like Forbidden Palace or Versailles) in Kiev, so I could take it out of the build queue there, actually build it for real in Siberia, and get a pile of failgold on top of the now reduced city upkeep. That had finally made my economy recover enough that I was able to actually start training workers and settlers and construct buildings instead of building wealth everywhere, allowing me to get rid of all the woods and replace them with farms, workshops, watermills and a few cottages.
I'm not sure when I switched my civics, but I think I did it rather early. I can't remember if I stayed in Republic, but I definitely switched out of Capitalism (ability to rushbuy which doesn't do jack if I have no cash to spare, and extra commerce for towns which was meh) to Agrarianism (+1 commerce for farms), turning my many riverside plains into 2F1P2C tiles, which was decent enough. With what little I could spare for research I made my way to Military Tradition for Cossacks and trade bait (there were a lot of civs, seeing as it was a huge Earth map). I got constantly pulled into worldwars because I just couldn't learn my lesson and stay out of defensive pact webs, but I didn't lose much from it besides trade opportunities. What annoyed me the most was the British Royal Navy blockading St. Petersburg again and again throughout the 19th and 20th centuries. I might have conquered a bit in Eastern Europe and settled Siberia during the 18th and 19th century, but I was mostly focused on domestic development and researching key techs. One thing to keep in mind is that inflation was absolutely insane, more than doubling all my expenses the entire game.
I made two more civics changes (in this mod Golden Ages don't remove anarchy, so I had to be picky), the first sometime in the 19th century to somewhat democratic socialism:
Republic (faster cottage growth, more great people birth rate, but high upkeep)
Representation (+2 beakers from specialists)
Industrialism (+10% Production everywhere, +1 Production from Workshops)
Central Planning (Double Production speed for Factory and Coal Plant, +1 Production per specialist, but high upkeep)
Secularism (no state religion, +10% research everywhere)
Levy Armies (military units consume food, less unit upkeep)
This period was marked by a sudden spike in research and supercharged (+1 Production, +2 Research, higher great people birth rate!) specialists, allowing me to catch up in tech and become a modern industrialized nation. I mostly picked Levy Armies because it was the cheapest option in the military civic category, and I didn't mind losing food because I wasn't actually training many units. I might have conquered a bit more territory here and there when the opportunity presented itself, seeing as I had so many redundant units lying around, but my main focus was on climbing the tech tree to Assembly Line, Railroad and Fascism. Why Fascism you might ask? Because it allowed me to switch my civics to this in the early 20th century:
Autocracy (+25% espionage,+2 happy from barracks, can draft 3 units)
Totalitarianism (no distance city upkeep, +1 espionage per specialist, no upkeep)
Industrialism (see above)
Central Planning (see above)
Secularism (see above)
Naval Dominance (+25% Production and +2 XP for naval units, double production speed for Drydock)
I picked Autocracy over Republic because it has medium upkeep instead of high. Totalitarianism is of course a
godsend stalinsend, seeing as I already had a whole lot of cities at that point and was planning to get some more. The extra espionage helped me steal some techs here and there later on. My economy was still communist, but now my Soviet Union had turned into a Stalinist totalitarian hellhole, but of course it was all for the geater good!
Naval Dominance might seem like an odd choice seeing as I had like a single productive coastal city (Leningrad), but most alternatives in that civic category (military) weren't appealing: Mercenaries are for civs with lots of money and lack of production, I have lots of production but a constant shortage of money. Standing Army would have given me bonuses for land military, but my inland cities had enough production and Marathon makes units so cheap that I had more land units than I knew what to do with anyway. Levy Armies, while cheap, wasn't an option anymore because I wanted my core cities that had finished all buildings and now just trained units to grow. Also my biggest threat during that period were the cursed British imperialists and I needed every little bit of naval power I could get to stand up to their ships. The 20th century saw my glorious workers' paradise turn into the biggest power the world had ever seen! After a few decades I had liberated all of continental Europe (most controlled directly, but a few parts were owned by my
vassals socialist brother peoples of Austria and Byzantium.) and a significant portion of Asia. While it took me a while to build up a navy, I did manage to hit the British where it hurts by liberating their oppressed colonial subjects in India (after
invading supporting some revolutions in Turkey and Iran) from their yoke. My biggest success however was completing the Internet, solving all the tech problems that had constantly plagued me for good. Like I said I was constantly on and off at war, mostly with the British, and by the 1950s I had enough of their shenanigans and just nuked their stupid little island before finally destroying them, splitting Ireland and Great Britain between myself and Soviet Austria.
It was time to start heading for victory. The Unique Historical Victory wasn't an option because the deadline for the first condition had passed before I even took over the game from the previous player, even though it was literally the easiest one of the bunch, expecially on Marathon. All it requires you to do is settle seven cities in Siberia by 1700AD, and while that might be an issue on normal speed if you actually care about settling good cities, it is trivially easy on Marathon because movement speed is lightning quick compared to the passage of time. And yet, my predecessor failed it.
Anyway, the only realistically achievable victories would be Domination and Space Race, and I decided on the latter because I had gotten tired of moving my fifty stacks of doom around. Note that the game at that stage kept crashing every turn because of memory allocation failure, so playing that game looked like this: Load savegame, nuke and conquer any troublemakers (which was fairly rare, but now and then someone somewhere on the planet was stupid enough to attack me), save, tell workers what to do and maybe conduct an espionage mission, save, tell cities what to build, save, end turn, save, rinse repeat. During any of these stages, but most likely after ending the turn while the computer was computing AI turns, the game would crash and I would have to restart it and load the most recent save. I think you can understand why I would only do the absolute minimum amount of work necessary and wasn't in the mood for actively planning and leading conquests.
In the late 20th century the situation on this Earth was like this: There are two superpowers, the New World Order (fascist America, FDR did his fair share of warmongering too and even tossed a few nukes at Australia and Latin America) and the Union of Soviet Socialist Republics (while I originally meant to switch to more democratic civics after I had taken care of England I kept postponing it and never got around to it and instead just stayed a Stalinist totalitarian hellhole until after I launched the spaceship), locked into a cold war. England and a few other locations are nuclear wasteland. The USSR and its client states reach from the Atlantic Ocean to the Pacific, from the Arctic to the Indian Ocean, from Ireland to Vietnam. The third most powerful independent civ on the planet according to the scoreboard? Congo. There are a few other weak and minor independent civs spread around the world, but they aren't important enough for the Soviet Union to care. If they were they would turn into radiation zones overnight.
Oh btw my economic situation had worsened again in the late 20th century because of Inflation. It actually became so bad that I started failbuilding spaceship parts (!) to fund my research of other spaceship parts.
Oh well, by the early 21st century I had finally completed the spaceship and launched it to Alpha Centauri, and after some loading->skip turn->save->crash I had finally achieved victory!
I think in the end I had more than 40 cities, and most of them had all buildings.
And that, kids, is how I repeated Russian history of the early 20th century, except it took me 300 years and was both more bloody and more successful.
If modded games don't count just disregard this whole epic story.
Edit: Would you look at that, this is my 7.000th post on this forum. And what a post it is!