I'm gonna do only BTVS.
Buffy
Best: season 2. She's strong, open-hearted, funny, sensual, broken.hearted, just everything. She matures and grows up a lot with major changes. So this is it.
Worst: season 6, no explanation needed.
Angel
Best: season 3. He was there for Buffy in every way, he made strong, painful decisions and gave Buffy all of his heart.
Worst: none. I love him in every season, his ability to place Buffy's heart and safety above everything else.
Willow
Best: I'll say season 4. She was much more aware of herself, she went through extreme pain and became stronger for this, made hard decisions, she met Tara and discovered a new part of herself.
Worst: I'll say season 6, we all know why.
Xander
Best: season 7. I loved how he helped Buffy a lot in the earlier episodes of the season, he was family for her in so many ways. Plus, the job and everything he grasped can be seen. He understood better himself and his place.
Worst: season 4. He didn't have a direction, a point in his life and he pretty much felt useless all the time.
Giles
Best: I'd say season 2. He helped Buffy as much as he could, gave her support, love and respect, he was starting a relationship with Jenny, he was balancing his life between watcher and man.
Worst:: season 6. Where is Giles????? :cry3:
Spike
Best: season 7. He finally learns something about himself and his soul and what Buffy needs.
Worst: season 6. Could that be more obvious?
Oz
Best: season 3. He showed maturity and forgiveness by giving Willow a second chance and he was just lovely as ever.
Worst: season 4. The Veruca thing and how he made Willow suffer is the only thing that come to my mind because otherwise that wouldn't be a worst season.
Anya
Best: season 5, when she's still way unaware of human ways, but she's happy and funny as ever.
Worst: season 4, still not completely in the Scooby gang and still unexplored personality.
Faith
Best: season 7. She finally gets a little bit more of how it is to be Buffy and she's way better reformed.
Worst: season 3, can't stand they ways she hits on Angel...ahhhhh!
Joyce
Best: season 5. Understanding and loving.
Worst: season 2. It's like she's suddenly waking up at what Buffy's doing and where she goes.
Tara
Best: season 6, ironically. She discovers herself as a person without Willow, she stands up and speaks for herself. She's complete.
Worst: I guess season 4, still not in the Scooby gang, still hiding behind Willlow.
Dawn
Best: season 7. She whines much less and she can find a place for her in the Scooby gang.
Worst: season 5. All the screaming and whining. OMG.
Cordelia
Best: season 3. She loses everything, she has to face harsh reality of the Xander issue and everything, it makes her grow a little bit (she still has a away to go)
Worst: season 1. The worst of Cordelia Chase.
Riley
Best:season 4. At least he's not completely stupid like in season 5.
Worst: season 5. His lack of personality is showed even more than season 4.
