Dragonite is much more useful and is only taken out by Ice type moves which are rare in Gen 1.I was going to say that's incredibly broken, but then there psychic types.....
I say Dragonite because in Gen 1 Mr Mime's Move pool was small and it didn't have that much coverage. Plus the go to Psychic Pokémon of the first gen were Mewtwo and Alakazam. Dragonite is much more useful and is only taken out by Ice type moves which are rare in Gen 1.
Well I was going to give a detailed explanation about the two but seeing as how you summed it up in about 3 sentences, i'll be going.