If it were me, I'd go by need then age, specifically highest need first, oldest server first within same need level.

The need level can be roughly ordered as this:
Critical need: Never start sage
High need: Does not always have a sage
Medium need: Fox doesn't always die
Low need: one/two group in GNW
High want: one sage bracket
Medium want: GNW not full
Low want: GNW doesn't have a meaningful qualifier.

So to those of you asking for mergers as if it's urgent, how high do you rank on that? Most post seems to indicate Low need level with "oh we only have 1-3 groups" that really doesn't mean anything, because if you always get a sage and fox always die, chances are it's because your server over-did group mergers and that's not really the game's fault. Sure, you have some need to merge, but it's not as high as others. So don't be surprised if other server who have too few people to even voice their opinion gets merged first.

My own server is of the "high want" level. So while hopefully a merger will happen on it, it's not urgent. and I'm not going to beg for it.

On a side note, fox might need a buff should mergers actually make server into low want or "good to go" level, as a high want server can finish fox in 10-25 minute. A low want server probably 5-10 minute. And a "good to go" might drop below even that. That makes the event pretty boring.