What's the easiest win condition and what the most difficult? Or does is really depends on which faction you play?

Supremacy with 1 other player.

Most of the time for me, the wonder victory is the most common, fastest, and easiest way to win.

From personal experience, if the stars align correctly with all strategic resources available and the main quest finished in a timely manner, Wonder victory can be short. Supremacy is also quick with a fairer consistency. Science and Economic can take a while longer since the main crux of either victory is accumulation of their respective FIDS. Elimination is tedious if AI's or other players straggle to outside lane systems in hiding.