the most reliable is to play a round-robin; that way you know how everyone stacks up against each other. unfortunately this usually results in way too many games considering that a round-robin between 8 people gives you 28 games.
more usually you'll have a general idea of who the best players are among your group even if you don't have an exact ranking. if you have that, you can do a few other things:
- pre-tourney seeding FFAs - as many as required. we only needed one for Kelowna 2012 between the top 4 players; you may want to do more, and do some among the lower ranked players as well, for more reliable seeding purposes.
- pools - organize players into as many groups or "pools" as required so that the top players are distributed evenly among pools. then, have each pool play a round robin amongst themselves and seed using the following results. we used this for Kelowna 2010.
- popular opinion or using the last tourney as your seeding information - just rank everyone manually and/or take it to friendlies if there are any disputes regarding ranking.
to answer your question, probably the most "official" among large groups of players is the pool system.