Buddys

Moltres is a hard hitting yet somewhat frail fire type and Metagross is over rated imo. If you don't have a %90+ Tyranitar yet, I would focus on it, a solid performer in so many match-ups. Looking into the future it has a mega-evolution too. Will be a mainstay.

Walk Beldum.

Walk beldum / larvitar / ralts instead.
For fire pokemon coverage you can go ham during next month's community day.

Don't walk Moltres. At 20km/candy, you are better off soloing tier 2 or 3 raids for rare candy. Beldum is a better choice. After I get 6 more T-tar candies to reach the final Jynx breakpoint, I may just walk a Beldum myself.

Someone earlier was advocating that they ONLY walk with legendary pokemon because everything else can be caught or hatched and will eventually receive more than enough candy for other pokemon anyway. If you receive enough rare candy though, that is a more efficient means of getting more candy for legendary pokemon.

I think pseudo-legendary pokemon are rare enough and strong enough that any of them are good buddy choices. That includes Dragonite, Tyranitar, Metagross, and Salamence. Although Metagross currently lacks any really awesome moveset or matchup, so I'm not too concerned with evolving one and I'm saving my good Beldum hoping for a moveset change.

Beyond that, any meta relevant attackers can be fine buddy choices. Especially if they are a pokemon that you do not see a lot of in your biome and you do not have a lot of nests available near you. Machop is 3km/candy. Geodude and Magikarp are only 1km/candy, so those can feel very satisfying.

Another option is walking with Eevee (10km and 2 candy) to evolve (while it is your buddy) an Espeon (day) or Umbreon (night).

Walking legendaries is a huge waste. If don't have a full team of good Tyranitars already I would suggest working on that. Beldum, Bagon, Ralts are other good options and if you have enough of everything that's out now, I'd walk Togepi or Tangela in preparation for gen 4.

And you shouldn't invest any resources or effort into any fire pokemon right now until we see how good Charizard's CD exclusive move is going to be.

I wouldnt walk with eiher Moltres or Beldum. Eventually you will hatch enough Beldums to be able to evolve one, plus it happens here and there, that you encounter one in the wild when its windy. Metagross isnt useful enough yet to justify using the buddy function for it. For Moltres all I can say is that: is one Moltres candy really worth multiple candy of a pseudo-legendary? Imo no, not even close. Rare candy are the way to go.

For buddy Id recommend useful things for the future like Tangela, Ralts or Togepi. Chansey and Tyranitar are viable buddys aswell.

Unless you have every top tier attacker already maxed out, I don’t see the point in walking a legendary. For a level 25 legendary, you’d need to walk 60 km just to power it up once. For that distance you can get 12 Larvitar/Ralts/Bagon/Beldum etc. candy. You’re better off using your free raid pass on lower level raids for rare candy and walking something more efficient as your buddy.

It's almost never a good idea to walk a Legendary. I honestly can't think of a single situation where walking a Legendary is a good idea - maybe someone else can, but this eludes me.

As for walking a Beldum to get a Metagross? It's... well, I did it. It was the only Pokémon (non-legendary) that I needed candy to Evolve AND I had a good one to evolve (a 96% Beldum hatched). I've got plenty of candies for every other Pokémon that I use regularly; my limiting factor is Stardust, not Candies. So walking the Beldum was fine for me. I'm currently walking a Bagon so that when I do hatch a good Bagon (my best is a 72% bleh), I can immediately evolve it to a Salamence. I don't NEED a Salamence, I just want to finish my Dex entries. So if you're looking at walking Beldum from that perspective, it's a good idea.

If you're looking to walk a USEFUL Pokémon - Ralts and Larvitar are top of the list - Ralts over Larvitar because Tyranitar is still a Raid Pokémon (unless that changed? I haven't seen a Ttar raid in a week or two... granted, with the scanners down I'm not seeing many raids at all, but people used to post Ttar raids at the same rate as Legendary Raids...) and there is a possibility of saving 113 candies by catching one (123 for evolving, then you get 10 for catching it). I'm considering Tangela and Togepi as well, though I don't have many good ones of either (all of mine are sub-90%) to really want to evolve right away into their Gen 4 forms when they are released.

I have 7 worthwhile Dragonites, however, the OP might not have 3+ and if so, maybe a few were Draco from the dratini day. Option to walk to try to max a dragonite(s)

Ralts: If you don't have one maxed out Gardevoir yet then definitely walk one of these, but you don't really need more than one (or two with alt moveset) powered-up Gardevoir. My #2 choice.

Beldum: I've found them hard to hatch so at least enough to evolve, they get fed in gyms often and have a unique niche but that niche is not really needed yet (Really it's only good against Gardevoir currently), if you DO hatch any that's more candies to power it up. Anyways, my #3 choice.

Salamence: I've found them spawning more than Beldum in Windy weather, mostly a Dex entry.

Dragonite: I'd avoid this because there are plenty of Dragons out there, if Kanto or Community Day events didn't get you enough candy already then stick with Rayquaza or Lati@s for this role.'

Tyranitar: This is really the #1 guy to be walking IMO: Nothing can do what a Tyranitar does best and there are a LOT of Psychic or Ghost types out there and in the future that will require a Tyranitar beatdown. The only downside to this is the fact that Tyranitar candies can be gotten easily from raids so its candy isn't that valuable, and Tyranitar raids have never really gone away unlike all other raid bosses. Also, if you're a high level, then there's no need to max out more than 4 Tyranitars so if you're thinking you need an entire team of them, know that they're bulky enough to usually outlast the timer by the 3rd or 4rd one - even 4 is somewhat overkill.

Chansey: A decent buddy if you want to max out a Blissey, but know that Chansey spawns have been increased for the past few events, and you don't need more than one Blissey powered up really.

Snorlax: Doesn't evolve so you don't need as much candy. Was a good (albeit low DPS) generalist for accounts without a lot of resources, but the ability to get good level 20 'mons from raids has mitigated this. I wouldn't bother, and they don't even come close to Blisseys in gyms.

Feebas: DEX entry, decent defender with coverage but I really wouldn't power beyond evolving it unless I got a super good IV one.

Rare candy for legendaries. At 20km per candy it's not efficient. I'm walking Tyranitar for now until I get a good gen3 thats rare and valuable.

I walk with Absol. How else am I ever going to get candy for it? He appears only in raids, but does so extremely rarely. I have not seen one in weeks and am actually not even sure that he still is an active raid boss.