TvP – A Terran’s view

Marines: Obviously great early game, and a waste of money late game. I'd rather be banking 3000 minerals than have 60 Marines, although Marauders are mineral-heavy enough that you shouldn't really be banking much.

Marauders: Basically the core of your army all game. Not that great against most Protoss units (Zealots, Immortals, HTs mostly) but a lot better than Marines by virtue of not dying instantly, and in general more versatile and efficient than other units against the Protoss ground army.

Reapers: Haven't really had any use for these guys.

Ghosts: Obviously EMP, but as I said I don't stress about EMPing single high templar. I usually get 2-3 Ghost Academies if it goes for a very late game and start using them for cloak+nuke since if you drop several nukes and engage at the same time it can have the same effect as drops--they can't divide their attention everywhere, so you are guaranteed to do some damage, and as it happens nukes tend to be a lot harder to spot and a lot more devastating than drops are.

Hellions: Good for mineral line harass, but surprisingly underwhelming against Zealot/HT compared to how you'd think they'd perform based on theory. I'd prefer to have pure Marauder for my main army in most situations, but I think I could experiment more with a hellion flank while my Marauder ball is engaging. Storm is also annoying since the fact that these are mechanical makes repairing damage a huge hassle, so you're probably better off suiciding them into random mineral lines once they take significant damage.

Tanks: Basically worse against everything they counter than Marauders are and countered harder by everything that counters Marauders, not to mention they lock you down mobility-wise. I don't see any reason to get tanks in this matchup past some early game tank drop shenanigans.

Thors: Pretty good, but really slow and once again they seem worse against (almost?) everything than Marauders are, largely because they can't kite. I don't make them unless the Protoss wants to try to abuse my lack of antiair and thus need some support for my Vikings against enemy Phoenix (which essentially never happens).

Medivacs: Keep a giant cloud of these above your Marauders at all times. They are essential for survival while constantly stimming and kiting, they rarely die, and they let you do drops when you get the opportunity.

Vikings: Work great in synergy with Banshees to snipe Observers, but I wouldn't keep more than a few around unless the Protoss is making air and/or Colossi.

Banshees: Decent in direct combat although the real killer is that they have their own set of upgrades that are kind of useless otherwise. If you can keep their Observer count down you should be able to force a bunch of cannons and in general cause pain for the lack of safe Protoss mobile detection until they start adding a lot of Phoenix to their army.

Ravens: Basically useless once HTs come out but really good until then. You might want to experiment with keeping an EMPed raven around to save scan energy when hunting Observers.

Battlecruisers: Seem awful post-damage nerf, but there might be some use for a couple Battlecruisers behind your Marauder ball if you catch them without any decent amount of anti-air. However, I suspect that anything that Battlecruisers can do, Banshees can do better.