sacred seal priority order?

I mean some of the seals' usefulness can be for niche roles. Fortify/Hone Skills for full support units, for example (like Eirika or someone else). Same can be said for Flier Formation, as it's best on a flier team. Generally, though, all of the Ploy skills are fantastic. Each of them is good in their own right. Atk +3 and Spd +3 or their equivalents are good general buffs. Speed Smoke will also be very useful. Imo, priority should be:
1. Atk Ploy (you're already at Atk Ploy 2 anyway)
2. Def/Res Ploys (depending upon whether you use physical or magic characters)
3. Spd Smoke
4. Panic Ploy
5. Atk/Spd+3

Atk/def+ seal is very useful as while I to doesn’t provide a solid 3 to either stat I found it has greatly improved the performance of a few of my units like Lyn, Both the dragon tikis, ephraim, caeda, Clair, hinoka, etc.

Next I would say speed smoke at least up to speed smoke 2 so that the debuff Is actually swinging such a large gap it can somewhat reliably prevent doubles.

Next would be your atk+, and Spd+ seals as they can straightforwardly bring a unit that extra offensive umph needed for Arena runs

Then finally I would prioritize the threaten skills FOLLOWED BY the ploy skills simply due to the fact that while both have the same 3/4/5 debuff incremental increase, a unit using ploy can do so from a safer distance back, while a threaten user is already within engagement range and needs every stat debuff to be going off in order to have a chance to survive said engagement. Reverse this last ordering if you aren’t using threaten skills on any of your main units.