Traditionally, where the Big Bad is the final boss for the season? No. Not by a long shot. But, I suppose there are a number of characters throughout the series that are more dangerous and developed than Orc or Vampire in period clothing that usually fills the show. Their story could fill several episodes, if not a whole season. This includes, any master vampire, which includes Dracula, characters like Sunday "the turned slayer," Veruca, Lower Beings, The Immortal, Season 4 Faith. You could call him a "Major Bad," but that sounds stupid. So yes, Dracula could carry an entire season, but, he didn't. So, not a big bad.