It really depends on what you need to do. If your project requires Lambda Expressions where you're query objects with a clear syntax, you should look at 3.0.
I am currently reading C# In Depth by Jon Skeet, and he takes the approach of laying out a solution in C# 1.15, then evolves the solution to depict the new and useful functionality that you get in 2.0 and 3.0. This type of progression would be a perfect means to answer your questions. The book reads well too so I am finding I am getting through it quickly.
