Note: I don't actually have a favorite game-- this is the first in a planned series of my favorite game of each genre
The game has two victory conditions. The first one is simply to wipe every other player off the map. Honestly, this is the victory I tend to get most often. The other condition is to research and cast the Spell of Mastery. However, the spell takes forever to research and longer to cast, and the AI has a nasty tendency to instantly declare war on you when you begin casting it.
All-in-all, Master of Magic is my favorite strategy game. It left a legacy in the realm of strategy games (Many of Master of Orion II's features were ripped directly from MoM), and Stardock has talked about making a spiritual sequel to the game.