Constantinople by a mile. Loads of money (i think i managed to get 6000 florins a turn from it once, and im a rubbish administrator), easy to defend from asian attack, and you control the entrance to the black sea. Also, if you block both crossing points over to asia with ships, A) Noone can blockade Consantinople or Nicaea, which is also a cash cow, and B) wandering diplomats all tend to get stuck, meaning you can train your assassins on them, bwahahahaha