In map without base defenses:
1)Power Plant (Increases cost of units)
2)Hand of Nod / Infantry Barracks
3)Airstrip / Weapons Factory

Remember that the power plant is the most vulnerable building in those maps but hourglass. Then, it might be destroyed first to increase enemy units' cost and turn its base power off-line then the base defenses will also be offline.

If oyu plan to place a beacon, place it outside the structure VERY near of it, then the enemy will loose time searching for it inside and might not have time to disable it in time.

Another good way to destroy the enemy structure is infiltrating a Hotwire or Technician and plant all C4 explosives on it. After explode the remote C4, plant the timed ones and use the proximity one to cover the time ones, then the enemy will be forced to disable the proximity ones first, loosing time; then the timed C4 explodes and the structure is down.

Hope you enjoy my tips!