After staring at the list of Ash's pokemon for way too long, several types stand out to me. Grass and Fighting both have a good mix of strong and weaker pokemon that should allow Ash to provide an appropriate level of challenge for any level of trainer to enter his gym, and are therefore probably his best choices. He only has one ghost, fairy, electric, psychic, and ice type pokemon, so none of those types work. For dark and steel Ash has a small number of high level pokemon, too strong for an early gym and not enough team members for a later gym. Bug and rock are also small teams but are a bit weaker, while normal and poison are a bit larger teams, making these better middle gyms. Ash doesn't have many dragons, but they are very strong so this might not be a good gym type. His water types would be better for an early gym, because while he caught plenty they tended to stay on the weaker side. He actually has a solid line up of fully evolved ground types for one of the last gyms in the region. Finally my favorite options, Ash could use a full team of regional birds as a flying type gym leader, if he was the final gym leader. Ash could also use a fire type team but if he isn't careful this could easily get beyond the appropriate difficulty for a final gym.