If i only had one chance to conceive a baby during my 28 day cycle, would the optimum time be the day before, on, or the day after a positive opk?

Day after OPK: You should ovulate (release an egg) 24 - 36 hours after a positive ovulation predictor kit test, so the day after the positive OPK is your most fertile time (the two days after is better).

Day of: Usually when an over the counter ovulation predictor changes color, the ovulation normally occurs the next day. Traditionally sperm needs to be in the reproductive track prior to ovulation. Therefore intercourse on the day that you see the change will probably be the most optimal time if the cervical mucus and the sperm count is normal.

Intercourse: I disagree! if you performed and IUI i would agree. The reason is that on the day ovulation the cervical mucus (under the influence of a early Progesterone rise) cervical mucus becomes less able to facilitate sperm transport. Having sex the day prior to ovulation will probably have more hospitable mucus and better allow for sperm transport.