I want to pass the ruby Pokemon with the monotype challenge because I have already done it in the sixth generation and I found it very fun. Any type is worth it except fire since in the sixth generation game I made it with that type and I would not like to repeat it
I'd say water is one of the best types (and likely the single best type), because there are many useful water Pokemon in Ruby and they aren't too reliant on TMs. Good options include Swampert, Pelipper, Sharpedo, Starmie, or Golduck. source Fighting and psychic each have several useful Pokemon, but they often rely on the same TMs (ie. calm mind, bulk up, and brick break) and thus don't work as well in monotype runs.