i don't think there's any choosing— ( ok so magikarps are really annoying ) at least in terms of training and battling! any pokemon is a great challenge, and everyone ( mostly? ) on the team doesn't discriminate— except for senkou who can't bring himself to fight girls with full power, even if they pretty much never take it easy with him, and shinya with his mouse phobia. in terms of catching, sasuke either goes for the fire types or pokemon that just look/will look really cool to him at some point. chinatsu has a somewhat natural aversion to water types, though, and if sasuke ever decides to catch one, she might try and steer him away from it!