Lady and the Tramp is one of the several Disney films that rely on the strength of the protagonists while the antagonists are secondary and basically used as plot devices. It would be difficult for me to rate it much higher than this personally speaking, but I do agree that the main characters are pretty strong. They aren't really my favorites by any stretch, but they work well together and the supporting protagonists are pretty likable as well.
I'll agree with you on The Rat though. Though the fight scene is brief and the "character" has no real personality, the portrayal of the heroes and the story of the film itself give way to allowing something that basically only represents evil to be quite effective. I also don't view Aunt Sarah as a villain, simply an overbearing, dog-hating old lady.
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Still smilin'
:>