I've been watching Buffy consistently since I was 11 years old. I still remember watching some Season 2 and 3 episodes when they aired though, which was several years prior to me actually watching the show consistently. I've been a Buffy fan for more than half my life! In my early 20s, I had a coworker who was also into Buffy. For the past 6-7 years, she and I have been texting back and forth discussion topics about Buffy and Angel (the shows), which include a lot of character development conversations and intricate dissections of the plot.
That being said, Spike is my favorite character. Has been since I was 11. This is due to the fact that James Marsters does such a great job with the character. Also, James Marsters has the same birthday as me, so my 11-year-old self was all over that! However, his character development from nerdy poet to self-sacrificing hero is one of my favorite stories. Without writing documents worth of details, he has the most interesting character evolution, and the most drastic, out of all the characters in the Buffyverse.
Tara is my second favorite, and I'm sad to see there isn't that much love for her. She is beautiful from the inside out, and her backstory is sad but empowering. I cry every time I watch the end of season 6!