Barracks > one Refinery opening for a quick Tech to Factory for Hellions to harass and then start Siege Tanks early for a quick defense opening that will make up for any loss in army value from the Hellions or Reapers.
This can extend to a fast expand + a macro Command Center (3) on long rush distance, but more likely is the need for fast Barracks (2 & 3) for the Marines and Bunkers needed to hold ground for the Siege Tanks.
Basic Build Order
- Hellions are fast enough to run into an enemy base and scout, if unsure what they are doing, just Scanner Sweep the part the Hellion do not scout.
- Do not have Hellions idle unless they are at Xel'Naga Tower, patrol them around likely proxy locations and look out for scouts.
- Only longer distances Marines and Bunkers can be skipped for faster Factories (2) (3).