The cheapest sensible way from your door to theirs.
Booking sites compare one airport to another, on one site. This plans the whole trip: every reasonable airport at both ends, the train or bus to get there, and a price watch after you decide.
Searches a full grid of nearby airports and days in one go, not one route at a time.
Adds the drive, train or bus to each airport, includes your bags in every price, and names who sells the ticket.
Finds hidden-city and self-transfer fares and says plainly what each risks.
Gives a verdict from price trends, and can keep watching a fare and ping you when it drops.