I'm not sure which is the best, the biggest one I saw looked like a giant pile of spaghetti or mashed up trees or something; I think it was a Tom Baker episode but I'm not sure... The Daleks were/are the scariest for me.
yes the Daleks were the best but I always thought you could escape them by going upstairs but then in remembrance of the daleks they had a new power what ment they could climb stairs
that's why in some dreams I used to hide in the attic, safe in the knowledge that they can only hover up stairs and not straight upward through little holes.