In Vanilla:

England - by far the easiest because of it's starting position, the overpowered Longbowmen line of troops who have AP arrows and bodkins that are unaffected by in-game weather, the general stupidity of the AI when attacking your lands (they always land near Nottingham and if you keep it stacked, which you should as it's your most important castle, you'll be fine). If you decide to exploit the Crusade mechanic and bribe the Pope to be your best buddy, you cannot be stopped and will be able to take the whole map before the coming of the Mongols.

Denmark - very interesting starting position with only one settlement. Powerful early game armies and a pressing need to blitz anything and everything. Jump in the fray and show the Pope the intimate details of a Danish axe!

France - very good unit Roster from the start until the end of the game. Blue in colour. In the middle of Europe and bound to fight Milan, the HRE and England. Hot French accents.

For complete fun, try a migration game. Start with, say Scotland, jump aboard a huge fleet and sail for Jerusalem. Then try holding it and expanding your kingdom from there, while letting your provinces on the British Isles rebel or get claimed by England.