1. Psychic - total pwnage in RBY.
2. Ice - Ice beam is the haxx.
3. Electric - Thunderbolt generally good except ground type.
4. Ghost - Gengar pumped special. Resistance to normal/fighting + shadowball effective against psychic.
5. Fighting - beats the crap out of tankers like milktank & snorlax
When I like the type, doesn't mean the pokemon has it as base type tough. I may give an unrelated type the move. E.g. Swampert Ice beam.
When it comes to mix types waterground is my favourite, because its not susceptible to electric attacks, and no supereffective water/ice. Hardly anybody uses grass moves so its good