I have to say England is the best faction in the entire game. As them, i conquered the entire map at the year 1267. Why is this? They have an absolute dominance of unique units than just own everything out there. Heres the list, off the top of my head.
Billmen
Longbowmen
Gallowglasses
Fyrdmen
Highlanders
Hobilars (not that great but still effective light cav)
Saxon Huscarles come from rebellions, as do Sherwood Foresters, which both rock hard
add to this local unique units ( ie Swiss Pikemen/Haleberdiers, Jinetes, Geonese Sailors) and you are unstoppable!
Im sure there are more. If you can eliminate France by the year 1100, you have a secure base from which you can go virtually anywhere. Fortify Flanders ands churn out Barques, and you can get Flanders up to producing 4000 gold a turn by itself. If Flanders is a well defended, Britain acellerates into an army generating machine, capable of churning out an early-age dominant army every three turns, mine consist of usually 3 longbowmen, 3 Feudal Seargants ( or fyrdmen), 4-5 infantry, just pick one, youve got such variety with them, 4 cavalry, often this will be feudal knights, and then a general. This machine can give you multiple thousands of men to just throw at the Germans or the Spanish of whoever pisses you off the most.
I steamrolled staight across Europe to Moscow with greater efficiency than the Third Reich, and destroyed the Mongols in 1231, by outnumbering them 15000 to 8000, though i could have taken them with a battle-hardened two and a half.
The Pope? No problem, Billmen absolutely pwn anything the Papacy can come back with, its usually a lot of cavalry, mixed with some nice foot knights, as im sure youve all seen.
The Muslims? an afterthought. The MTW Islamic world is a force to be reckned with on the battlefield, but they have a LONG windup, if you can get them on their heels before they get multiple Castles/Citadels, they're in the bag.
In conclusion, England is the best one faction there is, that is why I hardly play as them. I enjoy the Turks early age, French early age, and the Russians High Age. ONce as the Russians, my Westward Expansion was halted in central Germany, where my Western border met, of all people, the Almohads, who were throwing Faris around like they knew somebody. Man that was hard.