Personally I would say the Fallen, we would be seeing those four armed freaks a lot more than the others. The Hive only have a small portion of their troops on Earth and as humans we wouldn't be going to the Moon or Venus to say hello to the Vex and we most probably wont be going to Mars for the Cabal. So for me defiantly the Fallen as you would see them almost everywhere you go so for me its the Fallen.