I think the series has memorable villains, but handles most of them pretty bad.
Wesker was a good villain. Smart, cunning and then turned into bad-Neo. To me, his best performace is Umbrella Chronicles, where we can kinda see inside his mind. That being said, I dislike Wesker in RE5. His plan is silly, his attitude relating to Chris is stupid and by the end he was whispering about "Complete Global Saturation."
Alex Wesker also had a good start. I loved her as the "Overseer", the way we can only hear her taunts and all. It's a shame she was relegated to turned into a monster like every RE villain. And her monstrous form wasn't that much interesting.
Krauser was cool, because he was like an evil version of your character. Highly trained and capable, but his morals were completely twisted. He doesn't have much deep, but hearing his thoughts on the last chapter of Operation Javier was cool
William Birkin was pure chaos. There wasn't much into it. He barely had a purpose, but he was strong and dangerous and knowing how he got there was cool and I agree with
@bSTAR_182 on everything she said about him.
I honestly thought Simmons and the Family to be a wasted opportunity. A powerful man, inside the US government could cause problems to all characters in the series, and stir a lot of paranoia into our heroes. Shame he also was relegated to monster of the week. At least a T-Rex is a cool monster.
The one I dislike the most is Alexia. I can't synch into CV. Everything seem silly and over the top (not the action, but the plot...). I always called them Disney villains. I hope a future CV remake can change that. I gotta say though, Alfred in Darkside Chronicles is not so bad. I prefer the sadistic psychopath he is in DC better than the original.