Which Race to be in Starcraft 2

Terran:

Terran: Terran is an extremely defensive race. The race centers around creating a heavily fortified position in the early game, usually by creating a front door wall with supply depots, barracks, and/or factories. The Terrans than build up a large force unopposed and move out when the time is right.  Because Terran strategy is very straightforward and in the process of climbing the tech tree they unlock almost everything Terran is generally considered the "Easy" race.

If the Terrans have a weakness it is that they can only operate as many bases as they can defend at one time. Terrans are usually limited to only 1 or 2 bases. Their units are slow and largely dependant on positioning. They are superior in a chokepoint but weak on open ground.

Zerg:

Zerg: If the Terran race says "This is my base and you can't do anything to stop me from holding it" the typical Zerg response is "Ok, but I get the other 90% of the map". Zerg gets a large number of extremely mobile units. Speed upgraded Zerglings are lightning fast, mutalisks are fast and flexible, Roaches and infesters can travel while burrowed. The Terran fight in a chokepoint, like near their base, and the Zerg fight in the open ground of the center of the map.

The zerg win by out-producing, or out-macroing, their opponent. Good Zerg players usually fall into one of two categories. There are the players who build almost nothing but drones as much as they can, for as long as they can. At any one point they have just enough forces to survive for now, and they build much of their army after they spot the enemy army leaving their base. They build nothing but the best counter for the opponent's forces.

The other strategy revolves around taking an active part in preventing the opponent from being able to macro. These players use their high mobility units to constantly harass their enemy. Often attacking the opponent's workers with mutalisks and other surprises. Instead of bolstering their own economy they build fast units and stop their opponent's. Constant harass, mutalisks, baneling drops, anything to slow the opponent is standard.

If the Zerg have a weakness it is that all of their strategies are highly dependant on the player's skill. A macro heavy zerg needs to know exactly how large an army the opponent could have at any given time, and exactly how much of an army he will need to keep to stop that army. A harass heavy zerg needs unparalleled control over his units to keep them constantly moving and attacking. The zerg is often considered the "hard" race, BUT if you're willing to work with their play-style it can also give great results and be very rewarding at the same time.

Protoss:

Protoss: There is a reason I'm covering Protoss last and that is because the Protoss race is best compared side by side with the other races. The Protoss can play very aggressively, using warp in and a large number of gateways to quickly drop a strong force on the enemies doorstep. A front door closed by gateways and a zealot or two at the entrance can be difficult to break.

The Protoss can put up a strong defense compared to the zerg, but not nearly as tough as the Terrans. The Protoss forces are more mobile than the Terrans but can expect to be outmaneuvered by the Zerg. The Protoss are strong against the zerg in a tight chokepoint, but should fight the Terrans on open ground. They can expand to a new base early, or live off of 1 base for some time.

The Protoss doesn't really have a definite weakness, but it doesn't specialize either. Its better than zerg at things zerg is bad at and better than Terran at things Terran is bad at. Its a very well rounded race in general. Protoss are generally considered to be the moderately difficult race as the race doesn't require quite as much micro and knowledge as the Zerg, but he still needs to know how to be effective against an opponent using a given strategy.