Choosing buddy for distance events

I would choose 2 or 4 because like you mentioned the blissey is for defence currently and donphan would be a new defender and on top of that it a pokedex entry and well he is just really cool.

I would walk the blissey first until you have the candy you need, and then tyranitar.

I wouldn't waste time with abra or phanpy as they are only semi-rare and you can hatch them. And you said dratini isn't an issue.

I don't know if larvitars ever spawned where you live pre-event, but if they didn't (as is the case with many on this board), you might as well bank as many of his candy while you can.

Walking larvitar now can help you reduce the 95 candy deficit at least lol. Blissey you can walk at normal times (any chance you have a spare chansey? If you walk her you can't put it in a gym)

Will never walk 3 or 4.

If you aren't trying to fill a dex entry, I think it makes most sense to walk something that doesn't have evolved forms or at least ones not in your area. For example, I can occasionally find a Dragonite. With a pinap berry, that is 20 candies. With a Lapras I can only get 6, so I would walk a Lapras as candies are harder to come by.

In your case, I think Blissey makes the most sense. Alakazam isnt great for anything unless you are an expert dodger. Donphan is a cool defender, but only with the right moveset. I would walk to power one up, but I wouldn't walk a Phanpy to gamble on a moveset

It will take you exactly the same distance to get the candy for all of them. No matter what you walk first.
I wouldn't walk Abra, since you may get a nest. Phanpy doesn't nest, but it's hatchable. It depends whether you are in a hurry to complete the pokedex entry.
I would definitely walk Blissey for the candy first if you need the defender. But, as I said it will take you the same distance overall to get your chansey candy and evolve tyranitar. What varies is which goal you get first. So, as a consensus, walk what you fancy the most. If you are looking for completing 1-2 goals during the event walk Blissey and then Phanpy.

Always walk the mon which you already have good movesets. It's not worthy to walk just so you can gamble again.

I would never consider anything other than Chansey.

I'd start with the Blissey and get the 8 candies you need there and then try to put as big of a dent in to the Larvitar as you can before the week is up. The other two you can always walk later when distances go back to normal, but for this week I'd focus on the ones that are usually 5k/candy

If your Blissey can get in gyms with 8 candies, do that first...

Then, the Larvitar for sure...Even though you don't have a good one, stocking up the candy is wise so "when" you get a good one, you can max it enough to put in gyms...I've hatched 5 Dratini's since playing and just got a Dragonite so it's a great feeling if you get a good hatch and can instantly max it or max it enough...

I have stocked up on near 200 Larvitar candies now too. Just waiting on a hatch.

Have enough Snorlax candies to get a good spot as well and back to walking Dratini now in case I get another.

I don't walk Chancey since I have 2 maxed and want to "diversify" in case of meta changes/nerfs who knows...I also am doubtful that I can get another Chancey with 15 ATT so rather just wait for that to happen before walking.