There was a thread similar to this a long time ago (i.e. years), but it’s gone now.
Mr. Game & Watch (SSBM to SSBB)-This is insane. He goes from bottom tier in Melee to top 5 in Brawl. They decided to make all his aerials broken, his smashes fast and strong, etc.
Captain Falcon (SSBM to SSBB)-Goes from 6th best character in Melee to worst character in Brawl. They literally destroyed everything that made him good and gave him nothing in return.
Dictator (CE to HF)-Simple. He goes from best character to worst character. They nerfed his two broken moves (among other things I believe) and made him a little more even.
There’s definitely more I’m missing, and before anyone starts saying anything, yes, Smash counts.