Hi,
I am in charge of the pairings for a go tournament. I was wondering what kind of seeding was best.

It is a tournament with 5 rounds, McMahon style, with about 70 registered players.
For the top group, we rather follow the recommendations of the BGA than the ones of the EGF : 6 to 12 players in the top group.
We award prizes for the 1st 2nd and 3rd players at the end of the tournament.

Under these conditions, is it best to use Split and Fold, Split and Slip, or Split and Random seeding for players in the same McMahon group (espacially the top group) ?

Good. The EGF recommendations are bad (and totally out of line with every other recommendation, as seen on https://senseis.xmp.net/?McMahonPairing/BarTheory)

My personal preference for 5 rounds is 8 players if you can reasonably manage it. Due to the very strong field I recently used 12 in the top group in the EGCC anniversary tournament, and in the end 2nd and 3rd place both got 4 wins, losing to the winner (who got 5) and not meeting each other, which means that was too many IMO.



My personal preference is generally split and slip for the first round, split and fold for the second and later rounds, based on rating in the first two rounds, then SOS + rating after that. This is because it can be an issue that the bottom one or two players in the group are sometimes significantly weaker but added to make the group even, and I want to avoid the top contestants getting bad SOS due to an unlucky first round pairing. Giving them middle of the group opponents with split and slip avoids that.

Interesting !
Thank you for the feedback.