Well, the easiest is Terran, the middle class is Zerg and hardest is the Protoss.
Terran is easy because if you ever played some RTS you are mostly familiar with their concept: marines, snipers, tanks, fighter jets...etc. It's pretty easy to master, especially the
turtle tactic (and play for attrition), because only the Terran buildings and units could be repaired and they have superior air and ground defenses compared to the other races. They are the most balanced race in the game. Also, they have
NUKES! BWAHAHAHA!

There is no better feeling than hearing
"Nuclear launch detected!". Well, only if
you have the nukes
Zerg is fairly easy, and really use one tactic:
MOAR! They can pump out massive hordes of units in relatively short time, so if you mow down 100 Hydralisks, don't get your hopes up, because the next 100 should be just around the corner. Their secret is speed, early expansion and rushing. 80% of the time, a Zergling rush is responsible for finishing a game before it even starts. The Zerg is best used with "hit & run" guerrilla tactics. Their drawback is the weak units and buildings. No Zerg base can stand a full blown attack, that's why you should have more than one base. Also, the Guardian is essentially cheating, because it has a longer range of fire than any other unit or base defense in the game.
Protoss is the hardest. Their units, buildings and especially upgrades cost a fortune and they are slow and bulky as Hell. Building units and buildings is a hassle and takes forever.
But, they are insanely strong and versatile, especially when fully upgraded, a few Protoss units can turn the tide of the battle against insurmountable odds. They have regenerative shields, killer attacks and their air force is the best in the game. (You can't survive a full squad of Carriers when they are teleported straight into your base by an Arbiter.

) The Protoss rely more on the individual unit tactics rather than full squads or groups, so they require a different approach compared to the other two races. They are bit hard to master, but when you do, you'll be unbeatable. Plus, they look awesome
These are just my observations, hope that helps.
EDIT: you can never beat a Korean. Don't even try
