Nephlite - In my opinion, Neplite was probably the best badguy in the series that has played in the US so far. He is most definitely not the most evil or most powerful, but he showed something none of the other Generals showed. He showed he had a heart, even tho he himself did not know it.
Zoicite - Zoicite was Malachite's lover, and the killer of Nephlite. In my opinion she was probably the most evil of the Generals, but again, not the most powerful. In the Japanese series Zoicite was a man. I guess DiC didn't want little kids asking why two men were in love. Anyway, she meets her demise when she fails Beryl too many times.
Malachite - Called Kunzite in Japan, he was the most powerful of the Generals. He met his demise when Sailor Moon turned one of his attacks against him in the Scouts' assault on Beryl's fortress.
Queen Beryl - The first really big threat the Scouts faced, she was the force behind summoning the NegaForce and was in command of the four Generals that plagued the Senshi.