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Terran vs Terran
In most of the cases, terran vs terran will be a newbie match-up. Both players will be able to get outstanding defence and both of them will lack the attacking force. Most of the terran players can't do much without a few expansions, so preventing your enemy from expanding can win you a game, but it's hard, and you'll need to scout alot. The best way is the tank dropping on the ridge above your enemy's expansion place. If there are no ridges, cloaked wraiths can do very good, if not detected. The late game would come to battlecruisers - who has more of them, wins the game.
Terran vs Zerg
Most of the top players favour terran in this match up simply becasue terrans can stop zerg in any of their rushes. Scv's can easily eat up the 6pool rush, bunkers with firebats can stop any zergling rush, marines and tanks do very good against hydralisks, and stimmed marines with medics against mutalisks and finally, goliaths with upgraded air range have the same range as the guardians while doing more damage for less resources. The terran then have an edge - battlecruisers with valkyres. This combination of units can kill of most of the zerg in the late game. My other favourite thing is iradiating to death - building a few science vessels and irradiating any overlord you can.
Terran vs Protoss
Terran vs Protoss match-ups are usually won by protoss who either rush with dark templars, reaver drop or just mass carriers. Terran have a little edge over Protoss in early game (if not cannoned up). That is firebat/medic rush which eats up zealots, while you can bring some marines for the other units. After that, the Protoss get too much powerful. Also, there is a risky wraith rush that you can pull of to a bad cannoned-up player or a zealot rusher - by building one barracks, refinery, factory, bunker and two starports. As soon as you upgrade wraith cloaking, you can attack and hope for a win...
Zerg vs Terran
Even though this match-up favours terran, you can do very good with zerg as well. The first thing that comes in mind is the lurker rush, because most of the terran players neglect cloak detection against zerg in the early game. Lurkers are also very good for defence as they can kill marines in vast numbers. It is highly recommended that you win the game before your opponent get's irradiate. Otherwise you'll be in deep trouble. The second good opening for a zerg would be 3 hatch for massing zerglings, hopefully before firebats. The late game can be won by massive drops, hydralisk mainly.
Zerg vs Zerg
Zerg vs Zerg is a very tricky match up. While most of the players get 2nd hatch before the spawning pool, a 6pool rush can be very deadly strat. If both players go for 3 hatch zerglings or hydralisk, a few defensive sunken colonies can be very usefull. Mutalisk rush on both sides is also very tricky while a few scourges can turn the tide in your favour. So, when playing Zerg vs Zerg, early rushes decide the game.
Zerg vs Protoss
In this match-up Protoss tend to be overpowerd as the Zerg players can expand very early and acquire more resources for building advanced units. The best tactic used against a Protoss would be 3 hatch Zergling rush. While some players can cannon-up, you can bypass the cannons and hit the worker line very easily. Protoss have very much trouble defending any zerg rush, so whatever you do, it might be good :) Also, whatever you do, remmember that an early expansion is invaluable. If you let a Protoss too much and he gets carriers and corsairs, you'll be as good as dead (scourges can't charge on corsairs). If your opponet is getting templars, there are broodlings to compensate.
Protoss vs Terran
As I said, most of the Protoss vs Terran match-ups are won by Protoss. While most of the protoss players want to rush with dark templars, it's not as good strat as you can get. The massed reaver drop, by my opinion is the best tactic you can do, as it eats up any ground units (tanks too). If your opponent live to the late game, you can simply win by massing carriers and corsairs. Terran player won't have means from defending from this rush (well, most of them), while goliath with charon boosters can ruin your day, be sure to research distruption web. Battlecruisers with yamato can be annoying but remmember that they cost as much but that they are weaker that carriers.
Protoss vs Zerg
As mentioned before, Zerg favour this match-up. When playing against zerg player, it is very recommended to get the psi-storm as soon as possible as it can quickly kill most of zerg units with a single cast (but watch out for the queens). Also, dark templars in early game are invaluable for early defence. You can also surprise a Zerg player with a zealot rush. The reaver drop is also crossing my mind. Most of the zerg units have much trouble when charging against four reavers with observer. You can also pull of a droagoon rush or a recall. If you live to the late game and get the carriers and corsairs, you shouldn't have much trouble winning then.
Protoss vs Protoss
Doing a Protoss vs Protoss is also very hard. While in early game I recommend to go for offensive cannoning as it simply kills an uncarry player. Early zalot rushes can be very devastating while the dark templars don't do as good against other races. As you might have noticed I preffer the reaver drop and if there is the best race for reaver dropping then that is protoss. Most of the protoss units can't handle reavers if they don't have reavers on their own. Late game is usually decied by who has more carriers/corairs.