Many think that the missionary position (man on top, woman on bottom) is the best intercourse position to get pregnant. However, there's no research to back up that claim.
Any intercourse position that gets sperm near the cervix (or vagina, for that matter) can get you pregnant. Sperm will swim up the female reproductive tract with or without the help of gravity.
That said, if the woman is lying on her back when the man ejaculates, that may possibly improve your chances. There's no research on sexual intercourse to prove it, but there is research on IUI treatment that may imply position can make a difference.
IUI treatment is when specially washed semen is transferred via a special catheter to the woman's uterus during her fertile period. In one study, they found that women who lay on their back for 15 minutes after the sperm transfer had a 27% rate of pregnancy after three cycles. The women who got up right away after the transfer had an 18% pregnancy rate.
The Best Position to Get Pregnant May Be What Turns You On More
On the other hand, there's also research that says the more you enjoy sex, the more fertile you may be.
Studies have found that men who viewed pornographic films before masturbating produced more semen than men who did not. One study found that when the pre-sexual act is longer, more sperm is present in semen samples (also obtained through masturbation).
In other words, more foreplay and more sexual enjoyment may mean more sperm, and more sperm may boost your chances of getting pregnant.
(Please note that hot sex, as it were, will not cure male infertility due to low sperm counts. If anything, it may just give the still low counts a small boost.)
If the missionary position just doesn't do it for you -- but you're wondering if the man-on-top position for intercourse might be better for getting pregnant -- you can switch to man on top, woman on bottom, just before ejaculation.
However, keep in mind that a less-than-ideal position for intercourse is not going to cause infertility. While your Aunt Mildred may tell you that the reason you can't get pregnant is because you're "doing it wrong", there's zero truth to that statement.
Custers IM, Flierman PA, Maas P, Cox T, Van Dessel TJ, Gerards MH, Mochtar MH, Janssen CA, van der Veen F, Mol BW. "Immobilisation versus immediate mobilisation after intrauterine insemination: randomised controlled trial." British Medical Journal. 2009 Oct 29; 339:b4080. doi: 10.1136/bmj.b4080.
Pound N, Javed MH, Ruberto C, Shaikh MA, Del Valle AP. "Duration of sexual arousal predicts semen parameters for masturbatory ejaculates." Physiology and Behavior. 2002 Aug;76(4-5):685-9.
van Roijen JH, Slob AK, Gianotten WL, Dohle GR, van der Zon AT, Vreeburg JT, Weber RF. "Sexual arousal and the quality of semen produced by masturbation." Human Reproduction. 1996 Jan;11(1):147-51.