That depends on what you need and what your region is like, anonymous. Different terrain and different forms of transportation call for entirely different pokémon. For example, for an island region, it may be more important for you to have a pokémon that can Surf or Fly, at which point many of the ones that can learn either move are suitable, even if they may seem small. (You’ll be surprised at how much a farfetch’d can carry … or how it does so, for that matter … or how quickly.) By contrast, a mountainous region may require a pokémon well-suited for navigating rocky terrain (such as rhyhorn), one with high endurance (mamoswine), or one adept at climbing (machamp)—or alternatively, one that can fly at high altitudes (dragonite). For long distances across even, dirt fields, you may find it suitable enough to use any number of traditional riding pokémon, such as tauros, rapidash, mudsdale, arcanine, or dodrio, but if you were planning on getting through long distances over water, you may wish to have high-powered water-types, such as lapras or gyarados.
In short, it entirely depends on your needs, anonymous, but you can generally figure out what would be best by looking at a pokémon’s physical capabilities. If a pokémon is known for being able to endure certain types of terrain or if they’re known for certain types of qualities (such as speed or even their docile natures), chances are they’ll make excellent riding pokémon.