Finished the first 3, no sweat.. now I've reached the first fork. Looking for some advice on which path to take? Trying to get all the brutal achievements.
For most missions, the order doesn't matter, but there are a few key units that it pays to get as soon as possible.
Vikings are useful in several missions, so I think it pays to finish the colonist storyline as soon as you can. In particular, they can be used to knock out Protoss air in "Welcome to the Jungle" (they're way better than goliaths). They're also useful for getting the research in "The Dig".
Battlecruisers can also get help get through some of the tougher missions like "Engine of Destruction", so complete "Maw of the Void" as soon as it's available.
The Zeratul missions don't benefit from any units or upgrades obtained in previous missions, so complete them as soon as possible to get the research.
I personally found this very very helpful.
Mar Sara 1: Liberation Day -marines Mar Sara 2: The Outlaws -medics Mar Sara 3: Zero Hour -bunkers/turrets Artifact 1: Smash and Grab 4protoss(4) -maurader Colonist 1: The Evacuation 3zerg(3) -firebats - Covert 1: The Devil's Playground 3zerg(6) -reapers Rebellion 1: The Great Train Robbery 3zerg(9) -diamondbacks Rebellion 2: Cutthroat 3protoss(7) -vultures Artifact 2: The Dig 3protoss(10) -S.Tanks Prophecy 1: Whispers of Doom 3Zerg(12), 3protoss(13) - Prophecy 2: A Sinister Turn 3Zerg(15) 3protoss(16) Prophecy 3: Echoes of the Future 4Zerg(19), 2protoss(18) *Prophecy 4: In Utter Darkness 3 Zerg, 3 Protoss Artifact 3: The Moebius Factor 3zerg(22) -Medivac Colonist 3b: Haven's Fall 3protoss(21) -Viking Artifact 4: Supernova 4protoss(25) Banshee - Artifact 5: Maw of the Void 4protoss -Battlecruiser *Colonist 2: Outbreak 2zerg(24) -Hellion *Covert 2: Welcome to the Jungle 3protoss - Goliath *Covert 3b: Ghost of a Chance/Covert 3a: Breakout *Rebellion 3: Engine of Destruction 3zerg - Wraith *Rebellion 4: Media Blitz - Thor *Rebellion 5: Piercing the Shroud 3 Zerg, 4 Protoss - Final 1: Gates of Hell Final 2a: Belly of the Beast /Final 2b: Shatter the Sky Final 3: All-In *=option if not on a speed run. also note that if you save Haven, you have all research for zerg. i thought the 40,000 was more worth it since i never use psi emitter, and if its a speed run you'll probably do nydus all-in. Prioity Purchases: bunkers, comm centers, SCV repair, stim, medic heal, S.Tank, Banshee, merc tanks, merc banshee, merc vikings, merc battlecruiser, turrets, Viking, marauder, firebat. (in that order)
The easiest way to get all the brutal achievements is to cheat: do the whole thing on hard/normal, and get the best stuff, then, when you've completed it, go back using the archive and replay the missions on brutal.
You can finish them without having to complete the secondary objectives, and still have all the best gear. This makes the campaign much easier: trying to complete the secondaries on Brutal is a nightmare. Mind you, if you realize you made a poor decision the first time you went through, that's too bad.
I played the whole thing through on hard, getting all the achievements for every mission, so it was easy, once I finished, to just go back and replay every mission on brutal difficulty. Sadly, I often found it easier to beat it on brutal, than it was to get some of the achievements on hard.