Probably the best (cheapest) PRSA in the world?

[ClubMan]

Should I simply be going with the one with lowest AMC fees?

I would tend to prioritise this but other factors like customer service are also important.

Or is it more important to focus on the overall fund performance?

I would be inclined to go with something like a passive tracker (e.g. MCSI World Equity Index) so the returns should be very similar regardless of the underwriter.