While a subwoofer adds emotion and impact, and the center channel produces most of the dialogue, the speakers you choose for the front left and right channels are potentially the most critical. This is because these two speakers are primarily responsible for creating the soundstage, whether that be for a movie scene or an audio-only soundtrack.
Think of the soundstage as the audio created in front of you that matches the action or performance in a scene or on a musical stage. It has depth and width, and even height. In movies, it creates a sense of realism for the space in which the actors work. In music, you can distinguish the drummer situated in the back; the lead vocalist in the center; the rhythm guitar strumming lightly to the front and right.
When sitting in a room's sweet spot, a great pair of front left and right speakers can create this 3-dimensional soundstage all by themselves. When combined with an equally great and timbre-matched center channel speaker, the soundstage remains cohesive almost regardless of where one sits.