I am of course speaking about Starcraft races. I am very lousy player with all of them so take this post as a advice to worst players out there.
Nice race with great defense possibilities. If you like turtleing they are great and once you reach 200/200 your army is most powerful thing there is.
Really powerful race with great units for offensive play. Real problem is when you need to defend. There is not too much special abilities out there so there are not trouble to manage when you have a big army.
Insects that reproduce really fast. Their abilities are something not seen in any other game so playing with them is a really good fun. Units tend to get very specialized in late game so you may have a problems with deducing proper ratio of them.
Which one do I use?
I haven't decided yet. :)
Since all races are somewhat same (in regards with power, unit-wise they are totally different) once you get to know them, it all comes to personal preference. In that light, terran does not suit me - they are just too plain.
I played with zerg a lot and they definitely have nice air units but somehow I never use their special abilities to full extend so defence from terran army is a huge problem to me. Especially when terran goes around hunting my overlords and then attacking me with invisible wraiths. Although they are not meant for that, defense is (for me) much easier with them than attacking. But if you can attack early there is no force that can resist.
Protoss is my current favorite since it allows me great land and air army. Detectors are fragile as zerg's but my economy is not hurting as much when I lose them. There are also invisible units there so harassing other players just comes naturally. With successful harassing, there comes a victory also.
[2010-02-15: I decided. I will be the Protoss. :)]