Yeah I agree with Natasha. 'Scout' has multiple meanings and each benefits from a different armor.
If you want a spotter, the void sig is even better then stealth, as you get a bonus standing still, which you want to do with spotting. This overlaps with infantry, but mech spotters definitely have advantages in survivability which justifies the cost.
A scout mech meant to mix it up in close range, either with tag/ecm/c3 or shooting attacks, wants the most protection it can, as it wont have range so stealth wont help. So this means max armor of whatever type, and if you still have room/tonnage, then upgrade to reflective or Reactive* or hardened/ferrolam. As mentioned by others, if you are playing out of the battlemech manual, there is very little AE damage, so a non-melee light mech benefits from reflective. If you are playing with tactical operations or lots of aerospace bombs, then AE damage is the #1 light/infantry killer, so you want a non-infantry unit scout that resists all the AE attacks from mines, bombs, and artillery, so you want hardened or Reactive. Ballistic Reinforced only helps versus ballistic and missile weapons, so it doesnt help against artillery/bombs/mines (not counting artillery cannons, those are ballistic weapons+AE).
Sadly Reactive armor is probably the least used type in the game, and its main advantage, resistance versus AE explosive damage, is pretty unique outside of hardened armor. So while I think reactive would be a great choice for light mechs to dodge artillery death, you probably have to take Hardened or Ferro Lam, as those actually have canon units to pick from, if you want to make an artillery resistant scout mech.