Funny how this sort of threads continually turns up in any sort of game forum even though the answer is consistently 'it depends' or 'there is no single best so-and-so'
Firaxis would've done a very poor job in the game balance department if there were a single civ/leader that clearly dominated others in any and all departments, no?
So as always the answer is: there is no single best civ or leader; the relative quality depends on your type type of game, goals and playstyle.