I'm pretty sure it has to do with a combination of all the stats + bonuses of all the club members -- including the base str/intel, tutor bonus, RS bonus, misc bonus, etc. Contrary to popular belief, it does not appear as though successful party completion has anything to do with it (i.e. forfitting a party will not hurt your club rank). Therefore, clubs geared to high stat players and big spenders will almost always be at the top of the leaderboard. This is why you see a lot of premium party based clubs up there, because those attract spenders.
I've been told it's total main stats only but what Muschi says above sounds about right. I would like to point out 2 of the top 3 are potd and 6 of the top 12 are potd 🥳
In case, this can be add to the original one For the Starter Club Information taken from the FAQ (information updated around October 6,2022) “The Starter Club is the default club that all new players automatically join! The Starter Club is full of other members who are all just starting out on PIMD and is an awesome way for new players to meet each other and become friends. The Starter Club is always throwing parties so that players can learn how to party. New players receive rewards from successful parties here as well! Players are welcome to leave the Starter Club at any time, however once a player leaves, they cannot rejoin the club. Please note: You will be automatically kicked from the Starter Club after two weeks to encourage you to make friends and find new clubs! Again, once this happens you won't be able to rejoin, sorry!”