A personal example of Big Bad engineering:
Season One: Victoria Valdermar, and her father. Former Watchers turned vampires, out for revenge against the Watchers and a bit of an occult underworld powergrab as well. Victoria became a recurring nemesis, as she was the fun one. A shadowy reflection of our hero Milli (in her own way a rebel against the Watcher system, attracted to the same guy, diametrically opposed in style and manner) who got more troublesome as the series went along.
Season Two: Ziggy's family. Since Ziggy's player was leaving. More directly powerful, actually less evil. A lot more personal, though. Came complete with a face turn for one of the family members.
Season Three: Charybdis. A mildly experimental idea here - a Big Bad who wasn't actually Bad, a walking magical environmental disaster. There was an evil cult using her for their advantage, but anyway.
Season Four: Victoria again, and then Jake. A really experimental idea here - a PC turned Big Bad. With a bigger, badder Big Bad plan. Resurrect every vampire ever slain. Probably the biggest and baddest until Season Seven, and certainly the most traumatic. Yes, I watched Buffy Season Two and took a lot of notes. I don't think we hit that emotional high because it was all a bit complicated and maybe too like it, but still, worth doing.
Season Five: The Highmore Club, a secret society doing the "occult underworld powergrab" thing again, connected to one of the new PCs Lucian by their use of mystical artefacts. Then their power got grabbed by party sorcerer Matthew, making him briefly the second PC Big Bad in a row. This was a way of eliminating his exponential sorcery uberness, as well as muddying the waters of his relations with the group. The Club had originally had an agent in Cambridge, dashing romantic foil Thomas, but he became impossible to take seriously so this plot thread was abandoned.
Season Six: Initially the Order, militant monster-hunters, a fake Big Bad that then got massacred by the real Big Bad, Nimue and the Dragon she was about to raise to destroy humanity for the good of the environment. She was a stealth Big Bad who appeared as a romantic foil, something I had tried the year before. Amusing footnote: I cast Emilia Fox as a villainous Arthurian witch two years before Merlin did it. In addition, Milli's player went travelling, so Milli descended into Hell to rescue Jake's soul from evil tormentors attempting to use it to escape.
Season Seven: DEATH. And a massive cult of vampires. But mostly DEATH. I knew I had to go big, and Buffy itself had brought out The First Evil for its final season, so DEATH seemed like a good bet. It also meant that DEATH could get smashed up a lot and pop back up, obviously. Ultimately I had to keep him offscreen more than the best Big Bads, though, and I deliberately avoided the Caleb route to bring in someone who could take a punch more readily. Oh, and The First Evil blew up most of the NPC supporting cast in the middle of the season as well.