Jensen Ackles and Jared Padalecki both did an amazing job as Dean and Sam. Their chemistry together is what makes this show so special. Like any great show some seasons were better than others but even the lesser ones are still worth watching. The only real negative about this show is the ending.
The top reason why Supernatural became worse after season 5 in particular is because of certain staff changes that occurred at this point. From the start, Eric Kripke was the creator and main showrunner of Supernatural. He gave the show its dark, grungy ambiance and horror-centric themes.