Personally, I tend to sit at the front, slightly to the right: I get the fun of the seats vibrating whenever the conductor gets excited and starts jumping up and down, plus a good view of the cellos (an instrument I occasionally play). For concerts with a piano soloist in particular, the left side is generally preferred, because that's where you get to see the fingers in action.
My not-very-scientific impression is that the ideal spot should be at the apex of an equilateral triangle, where the two extreme edges of the orchestra form its base; then you're close enough to hear well, but far enough away to allow the instruments to blend. The reality would depend on the acoustics of the hall though.
Speaking of which, I once heard an expert on acoustics on the radio saying that the theoretical ideal position would be for the listener to be slung upside down from the ceiling; the reason being that we hear higher pitches better if they're on our right side, and vice versa; in other words, the opposite of the way the string sections actually tend to be laid out.