I think she changes in each season quite a bit
Season 1 - Quite naive and innocent led astray alot by the likes of Kelly and Tiffani, probaly her sweetest this year
Season 2 - Her relationship with Dylan is now in full swing she has matured as a character and isn't as naive she knows what she wants and what it will take to get it, stands up alot more to her parents
Season 3 - A breakthrough season for Brenda she becomes much more independant as seen in the Paris episodes the betryal of Kelly and Dylan really makes her less naive and tougher
Season 4 - The season where she made her biggest character change imo she matures and progresses so much this year she's almost unidentifiable now from Season 1/2 Brenda. Grows away from the gang and into her own path as an actress I love her character progression this season.
Its a shame we never got to see what stage Brenda would have been at as a person in the fifth season but alas it wasn't to be she left on a high though which was good. So I simply cant choose a fave season as I love them all but 2 and 4 are my personal faves