How to define author order of a collaborative paper where all authors have contributed equally? I am finishing writing a manuscript in collaboration with a dozen of colleagues. The paper is mainly theoretical, does not involved experimentation of any kind and is born from numerous discussions between all the collaborators (it is actually hard to tell when the project started). No one really took the lead in the project, at least not during the whole time (for meeting setup and so someone had to take the lead, but only temporary). In addition, the ten collaborators are coming from 6-7 different research groups and universities.

In such case, when everyone as more or less the exact same contribution, what is the best way to determine the author order on the paper, without making any diplomacy misstep? 

PS: in my field , alphabetical order is really not common.

PS2: we do not have time for a 25 games croquet tournament...
Since your field does not normally do standard alphabetical ordering, then its use here might signify that something unusual is going on. So you could in principle list everybody in alphabetical order, identify the corresponding author, and then insert a note in the acknowledgments that everyone contributed more or less equally to the paper.

Alternatively, in some fields, papers can be authored by "teams" instead of individuals—although all the individuals participating in the team are cited somewhere. This is more typical when the collaborations run to the hundreds of authors rather than a few to a dozen. If the journal you submit to allows this, it would probably be the fairest option of all.