Korhal and solar beam are my votes. The warp in reinforcements thing is nice, but generally I dont think 2 zealots and stalkers are worth the 50 solarite it costs to setup. Its nice for an opening move at the start of a mission to get the first units out fast, but otherwise? meh. I would rather melt bases (and faces) with solar beam. Good job on holding the line. To me the worst section is the back with the flying hybrids and the zerg guardians bombarding from long range. I found the best solution is to wall off the area leaving the ground open at the edge with a decent squadron of dragoons able to run forward to get into range and attack the zerg that can hit your turrets from outside their own range.