zerg is the hardest race to pick up and go with, but it becomes very simple around the master's level of play, and then you become limited by the simple engagement mechanics during late game fights. you basically hinge on terran/toss players to make mistakes so that you can capitalize and win engagements.
terran is the second hardest race to pick up, it becomes the hardest around master's level of play, and the skill ceiling is the highest of the three races. there are always opportunities to play better, and has a very harsh learning curve.
protoss is the easiest to pick up, easiest at the master's level of play, and hinges on "cheesy" or "all-in" type of play. it's got a very low skill ceiling, but very harsh on poor engagements. typically, in pvx games, the forcefields/unit micro of toss players sets the pace for engagements, and poorly placed forcefields can lose you a game quicker than anything.
people will disagree, but that's from my experience as a zerg main player, toss offrace, and terran enthusiast. i'd like to get better with terran, but it's a very hefty investment of playtime.
all in all, terran is the true skill master race of sc2, but a lot of people have too much "racial pride" to admit it.