Perhaps a bit atypical, but my best PhD year was the third/final year.
My first year was characterised by insecurity and fear, my second year by insecurity, fear and anger at the establishment. Then, all of a sudden, my third year was a smooth, easy ride straight into submission. This was quite a surprise given the rough ride in the first few years (where I nearly quit a couple of times). The time between submission and defense was horrible, though.