Would it be better to run Adamant or Jolly in Ubers? I feel as if Adamant will be the better decision to be able to deliver much more damage to the sturdier Pokemon here like Lugia, Giratina-O, defensive Arceus, ect, but I'm not sure if there's anything important out there that outspeeds Mega Mence at +1 neutral nature.
The only important benchmark Adamant misses out on is outspeeding Lati@s and other base 110s like Mega-Diancie/Metagross while unboosted. With the exception of Mega-Metagross, all of these are threats you would otherwise KO before they'd be able to OHKO back. Running Jolly makes this 1v1 in your favor, as the opponent has to decide whether to risk attacking and being outsped (thinking it's Adamant) or switch-out and give you potentially a free turn of setup. Prior to Mega, you would also be outsped by +speed nature base 90-100s like Rayquaza and Palkia though I'm unsure how prevalent they are in the current metagame.
Jolly being able to speed tie max speed Support Arceus is nice too, albeit a situation you probably don't want to risk. Assuming you are somehow forced into the situation, it does allow you to do stuff like Sub at the attempted burn. Arceus-Fairy and Rock force you out regardless though.
+1 252+ Atk Aerilate Mega-Salamence Return vs. 0 HP / 248 Def Giratina-O: 306-361 (69.3 - 81.8%) -- guaranteed 2HKO
+1 252 Atk Aerilate Mega-Salamence Return vs. 0 HP / 248 Def Giratina-O: 280-330 (63.4 - 74.8%) -- guaranteed 2HKO
+1 252+ Atk Aerilate Mega-Salamence Return vs. 248 HP / 252+ Def Multiscale Lugia: 116-137 (27.9 - 33%) -- 88.3% chance to 4HKO after Leftovers recovery
+1 252 Atk Aerilate Mega-Salamence Return vs. 248 HP / 252+ Def Multiscale Lugia: 105-125 (25.3 - 30.1%) -- 0.4% chance to 4HKO after Leftovers recovery
+1 252+ Atk Aerilate Mega-Salamence Return vs. 252 HP / 0 Def Arceus: 331-391 (74.5 - 88%) -- guaranteed 2HKO
+1 252 Atk Aerilate Mega-Salamence Return vs. 252 HP / 0 Def Arceus: 301-355 (67.7 - 79.9%) -- guaranteed 2HKO
Base on these calcs, I would probably run Jolly just for the advantages listed above. Adamant does have slightly more power, but it doesn't achieve any significant KOs from what I can tell.