It's hard to say hey. Depends a lot on the style of play that you adopt.
Protos is by far the strongest, but their unit cost a lot, and they are generally slow to build, because they have a large tech tree to work through.
Terran are your middle of the road race. Their units are weaker than the Protos in one on one fighting, but they are stronger than the Zerg units. They have a very simple tech tree, and their units are fairly cheap, although still more expensive than Zerg units. Their buildings do not need to be placed on or around anything, so you can build them anywhere you like.
Zerg are the weakest race "on paper", and their units are also not able to compete with either of the other two races one on one, if you compare them directly. They have some big advantages though. Their units are very very cheap compared to Protos, and their tech tree is also flexible, so you have a lot of options.
I personally like to play with Zerg, but they are a hard race to master because of the weakness in certain combat situations.