Arabian Empire on CIV 5, Pangaea map. Conquered half of the known world, ended with the capture of Berlin. Foremost military and economic power.

On a large map in CIv 4 I have at least 60% of the world as the USA. Two countries are stuck as my Vassals and I can't get rid of them. One country is spread out through a whole bunch of islands so it is a hassle to attack him.

There are two others who are somewhat difficult to fight, but my huge spending power is beginning to show as I slowly take them over.

Arcturan Empire in Galactic Civilisations II.

My custom race of mind controlling parrots conquering the entire galaxy in their http://i485.photobucket.com/albums/rr212/Qeros/terr.jpg.

But if you're going by the effort involved, my http://i485.photobucket.com/albums/rr212/Qeros/italyfin.png in EU3 was took quite a while.

Ah France, your post-apocalyptic evacuation to Cuba and California is as beautiful as it is futile. I imagine an epic tale, where the freedom of humanity in the face of the mighty new Roman Empire rests in the hands of the only truly independent nation left: the noble people of Borneo!

I play Civ 4 on a huge map with a mod that allows me to start with 25 civs (actually up to 50 but I like 25). Couldn't say the largest, but I've made lots of big ones. Then of course there's Space Empires and Galactic Civilizations...empires built in those 4X games are by definition much 'larger'.

Made a Chiss Ascendency take over an entire game in Civ 4 (yes, I renamed a Civ the Chiss Ascendency from Star Wars).

They conquered all on the big map with about 4 turns to spare. Of course I had a Domination victory way before then.

I'm not sure if it qualifies as 4X, but in my most recent Crusader Kings II campaign I played as the Karen dynasty, with my administrative center being in Turkmenistan (because the Samanids are dikes). I was able to conquer my way through the Russian plains and had my territories stretch all the way to Volgograd. I know it's not all that impressive, but man was it satisfying. My vassals revolted against me three times during my dynasty's reign, and I managed to keep my little empire together through all of it. Pretty tedious, but it worked.

In the end I had but one heir, and he died of pneumonia. Worst game over ever. =.= The game can be a [censored], but it's so addictive and fun.

Not really 4X, but in CKII I've managed to take control over the entire map and hold onto it.

Shoshone, earth like map, largest size, new expansion that over nerfed war before fixing it, slowest time setting, prince difficulty, I was even behind in tech tree, rebel armies on all fronts, cities in open arms, happiness penalty so high I can't even remember, and conquered 70% of the known world. People on civfanatics said it couldn't be done. They said no ordinary man had the sanity to complete a sea invasion from one continent to another with the tax and diplomatic penalties. I say I am http://www.youtube.com/watch?v=bBW_oT9Q0Yg, Emperor of the Shoshone. Burn it all to the ground.

Distant Worlds easily. Its by far best 4X Ive ever played. Well Civ is good as its so casual and easy, Distant Worlds is truly HC.