I think the best material in the trilogy is the material traceable back to Puzo's novel - the Godfather and the flashback sections of GF2. The remainder of GF2, I think, is a step below that - a script that's confusing in many ways but that's saved by the performances of Cazale, Strasberg, Gazzo, and Spradlin.

GF3? I watched it several months ago just to see if it had improved with age for me. It hadn't. I haven't seen the full Coda yet.