These days the game is very balanced. Some civilizations are more suited to some players, some to others. The game can be played with many different styles, from the bloodthirsty warmonger to the builder.
A good player can win in many different situations: from a cramped scenario, where land is at a premium, and war is almost certainly necessary to win, to a wide open scenario where building is the way to go.
A good player can also win with any civilization, taking advantage of each civilization's traits. If you find yourself playing the same civilization over and over, try playing a random civilization, see which one falls to you, and play with its strengths, learning something new.
For that matter, a good player can also win without any specific Great Wonder; some styles of play rely on acquiring certain Wonders, the Pyramids and the Great Library being the most popular. A good player has a balanced style of play which doesn't require these. If you find yourself 'needing' a certain Wonder, try playing a game without getting it, and see how you do.
Civilization is a balanced and interesting game, deep in its levels of strategy and complexity. To say that a single civilization is 'the best' is too simplistic in my view.
As for Deity being the 'only' level to play, well, different people have different levels of experience with the game. Some players do not have the time and/or compulsion to learn to play on Deity level; if they enjoy playing on an easier level, then the better for them.
Playing on the hardest level does take a certain amount of micro-management. Many do not have the time of patience for this. One should play at whichever level provides challenge and enjoyment for them.
-Sirp.