Honestly? It's quite the difficult decision given just how many there are, and the way each individual pokémon behaves. I've worked with quite a few types over the years.

[Really, between his time as a scientist and how he would travel, and then there was taking his hand in raising starters for new students at the Academy. All of the pokémon he had encountered over all of his years.

To narrow it down to just one type—]

I suppose it is even more difficult, given I never had to try and decide a single type to work with given I never wanted to become a Gym Leader. More of a casual trainer in that sense.