The guy who takes a girl's virginity is the most important in her sexual history, period. Being the first means he'll have the most impact on her views on sex, shaping her preferences more than any other partner can. He'll also always be remembered no matter how bad their experience was together. Girls will often discuss how lame their first time was, meaning they still remember it vividly enough and that it's important enough to be talked about.

Some may say that what really matters is being "the last", conveniently ignoring that there were many "lasts". The first was once the last, the second too, same for the third and so on but only the first is THE first, the one and only first (unless it was group sex)

Some may deny that the first is the most important but the truth is that being a woman's first partner is more important than being "the last" or "the best" and if you are your gf/wife's ONLY partner ever you'll be the first, the last and the best all at the same time.