Buddy

I don't think there's an objective answer to that. Depends on what you need.

I am walking Mew because I don't need candy for anything else right now ¯\_(ツ)_/¯

I had a Larvitar that I had walked over 130 km. But it was never my best and never got evolved. Eventually it got transfered despite being a wonder.

Now I have another at almost as many km. Larvitar are quite rare around here and ine cannot have enough candy.

Bagon for dex...simply because I have never seen a wild one and only hatched 2.

I know as soon as I have walked it hundreds of km's and got enough to add it to dex it will become a raid boss...so I figured I would make that happen sooner ;-)

I would go Larvitar if you don't have a team of 3 maxed out Tyranitars yet and have trouble raiding Tyranitars in your area. Alternatively, if you have Latias as your current Tier 5 Raid Boss now would be a great time to raid Tyranitar since you aren't missing much.

I find Ralts/Beldum/Bagon to be the best walking buddies, but another great option would be Chansey/Blissey if you don't have a good one yet. I'm only going to max 1 Bagon, 2 Gardevoir, and 1-2 Metagross so walking buddies is kind of a finite thing for me right now. Still, I can't justify walking a Legendary at 1/4th the reward.

Blissey

Currently walking Beldum, but the best long-run buddy is probably Larvitar, followed by Chansey. Events have made Machop and Dratini less relevant as buddies. Ralts deserves an honorable mention.

It was this one for a long time.

Spent a long time walking larvitar and beldums, but need neither anymore and if adventure week is cvomming will be a good time to catch wild larvi. Currently walking kyogre.

I'm with Togepi atm. Trying to prepare myself for 4th generation.

I had Mewtwo as a buddy but then realized I could just feed him rare candies and walk Altaria for 3 reasons:
1. Next mythical research might require evolving a swablu.
2. Daily quest might be the "Earn a candy walking your buddy" one and I'd get an extra swablu for it.
3. Might find shiny swablu.

Personally walking Chansey, still got a long way to go until my defender squad is where I want it to be.
Honorable mentions go to Ralts, Tangela and Togepi which I've all walked before and will walk again if I finish up with Chansey before gen 4 is out.