Scout perk tree

Reconnaissance (battle scanner bonuses) - Always
Lightning Reflexes (anti-overwatch) OR Holo Rounds (mark target) OR Disabling Shot
Smoke and Mirrors (support grenade bonuses) OR Hit And Run (1 free flanked shot per turn, can't shoot twice)
First Aid OR Deadeye (anti-flyers) OR Ranger (minor damage boost, pistol bonuses)
Concealment (ambush enemies) - Always
Precision Shot (large damage) OR Packmaster (item bonuses) OR Sprinter (+2 mobility)
Extra Conditioning (2 armor, 1 mobility, heal quicker) OR Lone Wolf (isolation bonuses)