The position depends on the kind of orgasm a woman is after. Rear entry, for example, is popular for tempting a vaginal orgasm since it effectively stimulates her G spot area. Woman-on-top, on the other hand, is known to be one of the best in providing clitoral stimulation (especially since she's in control) for clitoral orgasm. This is important to note since so many women do not reach climax during penetrative sex if it involves no more than thrusting.
With about 70% of women needing clitoral stimulation in order to orgasm, penetration alone doesn’t cut it. She and her lover need to warm up that hot spot first, perhaps engaging oral action (which many women will climax from first, only to have another one later). Keeping her hub of pleasure fired up with finger action or a vibrator during thrusting can also be critical in helping her to orgasm. Some women, too, have other erogenous zones on their body that could help them to reach the “Big O” when stimulated during thrusting, e.g., her nipples. The best way to find out what positions and types of stimulation are best is to simply ask the woman, since preferences vary widely.