In my opinion, there is no "best civilization."  There is no civilization that will beat another civ any day of the week.  It all depends on the player.  For example, if you plan on going all out offensive, and making a massive infantry army, then your "best civilization" should be the Goths.  For mostly water maps, try being the Vikings or the Koreans.  
But there are civilizations that appeal to most people's strategy and tactics of this game.  For example, the Spanish have fully upgradable infantry, cavalry, and have a strong religous area.  
My best civilization varies between the Huns and the Britons.  The Huns have one of the best cavalry armies, except for maybe the Persians.  They also have bonuses on their trebuchets, and I happen to be an expert when it comes to trebuchets.  The Britons are useful against civilizations that to not pack punches in the archery department.  For example, the Goths: though Huskarls are extremely resistant to archer fire, they are not immune.  FIre and retreat, fire and retreat.  It will drive the enemy crazy.
I'm not a big fan of triple layering walls, stacking up bombard towers, keeps, castles, etc.  The only reason for this is that most players will stack up the defense, and then just leave it.  People, no tower can hit  a trebuchet.  All an enemy has to do is get some trebuchets and rams, and you have yourself a pile of wasted stone.  If you build a wall, have a unit of cavalry or something mobile to take out the long range seige. 
As for navy, if you're facing the Vikings or Koreans, then don't bother.  
Also, skirmishers are very useful at times.  I know that almost everyone in the world is laughing at me for this, but when you face an army of hit and run plumed archers/longbowmen, skirmishers will make your day. 
Okay, that's my ten cents worth.  I have been playing off and on for about 3 years, so I still have much to learn