As repeatedly said, the 'best' Civ depends on the map settings. Vikings on a small archipelago are nearly unbeatable (well, maybe Dromons can do it), while Mayans rock on a large wet pangea. Also, the level you play also counts - Celts are great for up to Regent, Persia or Ottomans are killer Civs up to Monarch at least, but the Dutch will serve you better on the highest levels.
Overall, I prefer Dutch, Byzantium or Vikings, and China for pangea. Simply love the SEA trait -hey, even England is a really good Civ now!