There are dozens of leaders to choose from every time you sit down to play a new game of Civilization V, each with their own sets of perks and bonuses. Some are good, some are OK, some are terrible, but only one is the overall best, and that leader is Elizabeth.
The thing with most leader abilities in Civ V is that most are highly specific: they're generally only useful for players tackling a very certain type of game, have vague and unimportant bonuses or can only be used for a limited window of time. Gengis Khan's ability is only useful if you're attacking city-states (rarely a good idea), for example, while William's is so reliant on a single set of circumstances that it's rarely ever employed.
Elizabeth, on the other hand, gets not one highly important ability, but two. It's almost unfair.