The version that you require depends on the version of Minecraft you are using.
For 1.5.0 : Forge V7.7.0
For 1.4.7 : Forge V6.6.0
For 1.4.6: Forge V6.5.0
For 1.4.5: Forge V6.4.0
Those are the major releases found on the MinecraftForge.net Forums
I hope that helps, If you need further help...