steel since they are not available to early in the game< If you want a harder challenge I suggest something im calling typelock...... once you beat a gym you are not allowed to use pokemon of that type (I am not counting secondary type), so after the first gym, no more electric types, after the second no more grass......etc..... at the begging it will be easy but after the first gym 36 are unavailable, 102 by the second, 166 gone from the third, 189 forth, 217 fifth, 240 sixth, 333 seventh, 383 eighth, 411 ninth, 436 tenth, 484 eleventh, 514 gone from beating the twelfth gym leaving 207 pokemon 23 probably unavailable at the time legendarys leaving 184, only way we will be unlikely to finish is if the 18th gym is flying because I doubt anyone will use only tornadus and five noiverns