I'd go with Beldum and grow it into a Metagross. Metagross is extremely strong, can levitate/fly to a degree, its psychic and communication between us would be easier, and it has the mind of a super computer able to make calculations that I can't with my feeble little human brain. For me Metagross would be my pick for a Pokémon journey or battle, but if I was in the real world or wasn't looking to compete against trainers, I'd go with Glaceon because its my favorite and it can cool me down when I get hot.
Jackass answer: Ditto, because it can be every other Pokémon and thus every Pokémon I could ever need. Actual answer: Probably either clefairy, audino, or tropius. Clefairy and audino are my favorites, but it's a tough call between them. On the one hand, audino is a giant fuzzy fairy rabbit who can detect more Pokémon to catch and serve as a Pokémon nurse on the go. On the other, clefairy is an adorable pink marshmallow moon cat that can murder dragons, and honestly, how can I resist something that adorable and that potentially hilarious? Like, clefairy is like the cat from every cat video ever made, only pinker and endowed with magical powers. It's perfect. But then you have tropius. Tropius is neither my favorite nor even on my top ten, but I'd probably consider taking one for a trainer's journey anyway. Why? Because it's a mode of transportation with free food growing out of its head. Of course I'd consider it. But really, more than likely, I'd probably go with clefairy. I mean, come on. The magic fairy cat videos that would come out of that...!
I would have to go with Lapras or Staraptor! Lapras is not only my favorite Pokémon, but it would be extremely convenient for crossing water, it's a great battler, and it's SOOOOO CUTE. Staraptor would also be handy because it flies and I could comb its emo hair all day.
I mean Kecleon and Salandit are my favorite shouldermons but if I had to pick a proper travel companion and only have the one? I would go with Torterra. Enough room to chill on, shade whenever you need and a chill personality.
If I had to choose just one, Espeon! Not just because it's my favorite, but a psychic type would always be handy to have while traveling. It can also sense incoming weather, so no getting caught in flash storms. Not to mention it would make an adorable pillow to lay on at night!
I'd stick with Pikachu since that's been my first exposure to Pokémon. I think it was pretty cool how it traveled with Ash in the anime and that's kinda how I see stuff going if I were to travel with a Pokémon. I guess from a realistic sense, I'd choose something big and flying type for convenience so maybe Salamence?
I would go for an Eevee and evolve it into an Espeon! I have always personally headcanoned Espeon as being capable of telepathy and that would be incredibly nice to have my Pokémon watching out for my back every day.
Blaziken, obviously. Wouldn't be beneficial to me in many ways, but it's the Pokémon I'd want to travel with, taking on any and all challenges, win or lose.
Charizard, because I could fly everywhere and I would always be able to cook my food. That and charizard is my bro for life.
Swablu or Altaria, easily. They're fluffy, adorable, good companions, can help you get to sleep, can wake you up, can somehow carry you with Fly, and can destroy someone's house with a couple of move spams.
I'd probably pick Croagunk as my buddy traveling companion. I got emotionally attached to Brock's Croagunk, and it has just become my favorite Pokémon since then. (I also used to have a small plastic Croagunk toy that I would keep in my pocket and fiddle with when I got anxious, so now I also associate Croagunk with something that grounds me and calms me down).
Half a year later and I think I would change my pick for Gardevoir. Not only is it a powerful psychic type, but if you could get it attached to you, there is no better companion.
I would choose a pidgeot. They are big so they can help to scare away people and I don't have to be annoyed most of the time. Also, they can fly so I can travel everywhere I want