The German PzIc is awesome, with the MG upgraded gun. It hauls ass and lays down a withering fire of 40 rounds at a clip. Plus the reload time is not bad.
The American Stuart and Locust are also very quick and nimble. Plus, the upgraded 37mm AT gun is accurate with good penetration. The alpha damage is not high, but the DPM (damage per minute) is sweet due to the rate of fire.
Two other light tanks, higher in Tier, are the Chinese 59-16 and the French ELC (I think that is its name) The 59-16 is fast and when you upgrade to the auto load gun (with the 5 shell clip and good reload time) you can really mess with enemy arty. The French ELC is also able to disrupt an enemy advance like no other.
Plus, if you outfit these light tanks with cammo nets, they are all good at passive spotting for your own arty and TDs. Remember, if you spot an enemy AFV, and one of your teammates kills that AFV, you will get one half of the experience points and credits that you would have otherwise garnered had you killed the tank yourself. There are many ways one can contribute to a team's overall success ... and get the win.