I will probably make a comprehensive guide later, starting off with some terran counters vs zerg first. Will do other combinations later.
Terran vs Zerg (main units)
-Zerglings: Against early zergling rushes, walling off is a great counter. With some SCV's repairing, marines can freely unload hell on zerglings from safety. If the zerg is going straight up zerglings, make him pay with either helions or reapers. Helions do well vs zerglings early on because they are faster and can kill several lings at once, but be careful after metaboost. Reapers with nitro packs don't work as well as helions, but are still great, able to 3 shot lings and drones, so a group of 3 can pick off several drones before being noticed. Siege tanks can be used to, but after the 1.1 patch they only deal 35 damage, so still effective if they have no armor upgrades
-Roaches: Siege tanks can deal massive damage to clumped roaches, mixed in with the standard m&m, roaches shouldn't be a problem. If you don't have time for siege tanks, marauders are a hard counter to roaches, with stim packs and concussive shells, a group of micro-ed marauders will easily deal with roaches.
-Hydra: Siege tanks do well against hydras because of the hydra's low hp. Even after the patch, a group of siege tanks will provide great support to defend against hydras. If you don't have siege tanks, then marines are another alternative. They aren't a great counter, but for the same cost, 3 marines will easily kill 1 hydra, i recommend upgrading marines attack, stim packs, and shields if you are going to use them tho.
-Banelings: Siege tanks! They wreck banelings, if you see them coming, quickly pull your troops back into siege tanks range and enjoy the bloodbath. If not, reapers deal good damage to banelings (keep them safe tho) and marauders don't take full damage because they aren't light
-Mutalisks: Frustrating to have a mass of mutalisks swarm your base when you are unprepared. Even tho vikings dont deal full damage to them, they still work well vs mutalisks. Thors are soo good vs mutalisk because of their high damage and splash to light air, but beware if they use the magic box, then thors aren't so good.If worse comes to worst, stim pack marines can deal with them, but the jumping attacks from mutalisks can own marines too, so marines are just to deal with them short term.
-Ultralisks: Banshee and BC's are great because they are air obviously. If you only have ground units tho, be careful to spread them out as much as possible. Marauders and thors do well vs ultralisks because of their high damage to ultralisks.
-Brood lords: Vikings dominate brood lords without taking damage in return. BC's are also good as well as PDD and Seeker missle from ravens. Thors can work, but not recommended. Marines should be avoided too, unless you can quickly stim and run under it to snipe it.