The old best player score was roughly:
roleplay score = xp / age
Because the best player was considered the one who spent the most time earning XP in game and not idling.
Now that we also track achievements, we simply also multiply by the achievement score:
roleplay score = xp * achievement score / age
The reasoning is that earning achievements means you are a well rounded player, living a full life in the game.
How about this achievement score?
It's complicated! Not all achievements are as easy to get as others. The best way to know how much an achievement is worth, is to look at how many players have really got it on the live game.
So, an achievement which 4 characters have reached is worth 1/4. An achievement which 100 characters have reached is worth 1/100, and so on ...
Then we just add up all the scores for the reached achievements of a player, so for a player who has reached four achievements, where the first achievement was reached by 277 players in all, the second by 3 players, the third by 1045 players, and the fourth by 22 players, then 1/277 + 1/3 + 1/1045 + 1/22 is the total achievement score.
If you have a calculator to hand please feel free to work out your own roleplay score and compare to the website. To compare accurately, please add a couple of constants we need to use for the arithmetic to work:
roleplay score = xp * (achievement score + 0.0001) / (age + 1)
Why the constants? Well, the bonus minute (age + 1) is because we do not want to divide by 0 for a brand new player! The (achievement score + 0.0001) is so that people who have no achievement score at all may still get some ranking.