I'm a person that likes to cover alot of ground quickly without the need to slow down so I'm usually on an assault kit with a squad sprint spec
Consider also that there will be time when your other squad mates will use the same spec as you for some reason..It's annoying I know but switch to another spec so you even it out
Please realize that the class themselves kinda sets what kinda spec should be added on-assault( grenade or sprint), engineer( flak and explosives), support( ammo, suppression fire, and suppression resist), and recon(ammo, sprint, suppression resist)
Other than that
Conquest- explosives and sprint
Rush(atk and def)- will vary depending on the map but I primarily use ammo, suppression fire and suppression resist
Squad dm-frags(idk why but it helps in dm haha) suppression fire and ammo
Squad rush- sprint, suppression fire, suppression resist, ammo , and expolsives
In my opinion all specs can be used to its out most potential in any game mode or map, but this all from my opinion