Do you know where the G-Spot resides? Its within the pussy about 2 inch inside on the frontal wall. The best way if you ask me, I would say the best would be going for oral sex, take her clit on your lips and insert two fingers and rub the frontal wall of her pussy.
On sex positions, its better to make her feel on both facing position. Reason is, a man's dick has a head or glans, the edge of the glans is called corona. On the corona, top part got the steeper edge than the bottom. Corona gives a better rub on G-spot on facing position. It can be missionary, she sitting on top or standing. On rear entry, the bottom of corona, which is frenelum is a flat one, the feel is not the same.
I would suggest if you want best G-Spot stimulation, than either go for oral with finger stimulation or go for woman on top, she would be able to control the depth and pace much better than you.