Pokédex progress

Set any of them as your buddy except charmander. Any of the other three are a good choice. As for Charizard... get out and play for a few hours before the current event ends. You'll have him in no time.

Right now, I am waiting on Dewgong (have 45 candies) and Muk (I have only seen two Grimers, so have a long way to go.)
Also, I am missing the regional pokemon other than Tauros and two of the baby pokemon. I have hatched at least three each of all but one of the babies I am not missing -_-'

There are still a couple more days to hunt for Charmander candy, so I would probably start with either Omastar or Dragonite as your buddy if I were you.

I completed the original regional Pokedex just before Christmas, and finally hatched the last baby last night (Magby). Technically I suppose I've got Togetic to go, but I don't think it's worth walking the last 12 candies.

Omastar involved walking (used the Halloween event), Muk involved walking and Charizard involved walking over 100KM (typically with the current event I'm catching more Charmanders than either of the other starters).

I'd walk Dragonite - even if you catch/hatch another Dratini before you've got enough to evolve the candy will still be useful. Alternatively walk something you're using now (Snorlax/Lapras?) to get shorter term benefit.

I have registered only 2 babies out of 7+Togetic. Have all else.
Last pokemon I registered was Chansey, found in the wild, about a month ago, when I was level 33 (luckily I found another and then hatched one).
I obtained quite late too those pkmn you mentioned.

I suggest you to set as buddy the most gym defensive competitive pokemon:
(In order of priority)
Snorlax
Lapras
Dratini

Needing 9
Alakazam- 45 candies to go
Dewgong- 47 candies to go (never seen in my city, caught on travel)
Chansey
Gyarados- 76 candies to go
Lapras
Porygon
Dragonair- 4 candies to go
Cleffa
Togetic-31 candies to go
Magikarp as buddy, since gyarados is the most usable between the evovables.

My regional pokédex was completed "again" three days ago, I hatched a third togepi and evolved him into togetic. The last one I needed before babies was a Lapras (I have hatched two on the last 2 months, just as an addition, both came with stupid Dragon Pulse of course, my usual luck...xD)

As for your buddy I would choose Dratini for sure, it is a long way to go yes, but for my experience it is better to choose top tier mons as buddy.

I walked a Charmander for some weeks, then 2 migrations ago Niantic made one park near home a Charmander nest, so during 2 weeks I got all candies needed and more making my previous effort almost useless.

After that I also walked Grimer for some days, and when I had 3 candies left a 2k wild Muk appeared on a random street on my way to work, I was really shocked...So my advice, walk Dratini and you will never regret, it is the most usable of them and even if you get more candies than you need or find a wild Dragonite there is no problem with that because you could take profit of your walking candies to power up.

Chansey is the last one I have left to find. In the last two weeks, I have hatched my first Hitmonlee, and evolved my first Machamp & Charizard.

I have 29 to go in my "wild-caught" pokedex. I'm still hoping to catch a wild Charizard before this New Year event ends. I've already caught four Charmeleons, three Venusaurs, and two Blastoises, not to mention numerous Ivysaurs and Wartortles, but no Charizard.

Tangela is the only missing pokemon for me...

5 and a baby here (& 3 globe-trotters). I have only seen one of them in the sightings (pory).
Dragonair
Dragonite (85 candies needed)
Lickitung
Porygon
Hitmonchan
Igglybuf

Wow I'm surprised you found Dratini that rare. They're all over the waterfront by me.

I just checked and, what are the odds of this? I have caught AND seen the same exact # of Dratini and Eevee, lol

(Barely any of these hatched, BTW. Maybe 6-7)

Just need Muk for Generation 1 regional, 14 candies to go, been walking it for a couple of weeks now. Also need most of the babies, only have Togepi and Smoochum.

I'm doing a mass evolution tonight and after that I can have a full Pokedex, including the babies (apart from Togetic).

I will however probably not evolve my Omanyte because I'm waiting for a better one.
So after tonight I will miss Omastar (even though I have the candies for it) and Togetic (need 12 more candies).

As for which Pokemon you should set as Buddy, I'd go with one that you rarely see in the wild.

I'm am missing dragonite (8 candies to go after walking 253 km with my only hatched dratini).

I don't have muk in my pokedex, but I walked with the only grimmer I've seen for 141 km, and I'm ready to evolve, just waiting for a good grimmer.

Also I haven't hatched elekid, and togepi, but I really don't give a damn about baby pokemon