Bowser, DK, Dedede, Charizard and Ganon. I'd classify them as tanks solely by their weight. They are the only 5 characters with weight of or above 110. (I don't really count Shulk's Yellow Art or moves that give armor)
I do think there is a second category of off-tanks or bruisers if you will. Characters such as Samus, Wario, Yoshi, R.O.B. etc. I'd prob classify them as characters between 100-109 weight.
To put in wrestling terms I'd view 110+ as super heavyweights, 100-109 as heavyweight, 99-90 as midweight, 89-80 as lightweight, 79-70 as featherweight, and Puff gets her own weight class as Puffweight bc she's THAT light.
I personally only view the super heavyweights as tanks but since that's only 5 characters I could see people viewing the heavyweights as tanks as well. But since that would include characters like Cloud and Captain Falcon who have speed it's harder to view them as tanks bc most games have tank classes as slow or low mobility champions/characters.
In Melee I'd say Bowser, DK, Samus, Ganon and Yoshi. For similar reasons as Wii U. They have the heaviest weight in that order. There is a gap between those 5 and the rest of the cast as far as weight is concerned. Even though there is also a gap between Bowser/DK and Samus it's impossible to not view Ganon as a tank so I feel obligated to throw Samus, Ganon, and Yoshi in the same grouping as Bowser and DK.