This is a bit difficult to answer, I’m afraid. It really comes down to the terrain of your home region. The best partner a ranger can have is one that can work closely with them no matter the terrain, particularly one with abilities that are both versatile and useful. After all, your partner is the only pokémon you’ll have on a long-term basis; you’ll need to choose one who can handle even the toughest assignments in the most extreme conditions.
Luckily, ranger schools across the globe will offer a choice of partners (or starting partners) to graduates. As you’ll need to attend ranger school in order to be a ranger, you’ll have a less overwhelming selection to work with (as opposed to, well, the entirety of possible partner pokémon in the world). This selection varies from school to school, and it’s often a point of consideration for aspiring rangers who are trying to decide which school to attend. (Of course, the other points should be the quality of education, who’s on the staff, location, and all the other considerations one would need to mull over when choosing a school. Do not choose which school you attend based on offered partners alone.) Additionally, there will also be advisers who might be able to guide your decision, and don’t be afraid to consult your teachers for advice while you’re there.
If it helps, though, electric-types such as pachirisu, pi(ka)chu, and plusle or minun are often popular choices. The capture styler has a limited amount of charge for each capture (that is, you need to let it recharge between captures, and there’s a chance you’ll run out of charge before you can successfully capture a pokémon). Electric-types, however, can have assists and field moves capable of recharging the styler. Even a small amount of electricity from an electric-type partner can be the difference between a successful capture and failure. Hence, many rangers prefer having an electric-type close by, just in case.
Best of luck, anonymous!