Which Snorlax to Power up

None. Their CPs are far too low.

20 candies is not going to get you very far with any of them. You can only get the lv9 or 10 up to lv15 where it will be about 1300-1400CP. Hold off and wait for more candy and/or another Snorlax with a higher starting level.

Wait to hatch one at Level 20 at least.

None of them.
Accumulate candies and dust.
When you have 150 candies and 200k dust. Ask the question again.

None of them at that CP, Snorlax are too rare and getting the candies to get them to a good CP will take too much effort, just wait to get a higher CP one with reasonable IV's, it can take you time but you will regret in the future if you hatch/catch one at around 1700 CP or even more.

None of them, you will not have enough candies to buff them high enough because they are so rare :')
You will save a lot candies and dust if you wait for a bigger one.
For example i have only one "big" Snorlax, 1900cp unbuffed. It has Lick/Hyper Beam, i have 49 candies, and i am waiting for a better Snorlax. Lick is bad for defending, and i dont like using Snorlax as an attacker. When i find a "big" Snorlax with Zen Headbutt, i will use my candies then :)

Start walking with one of them as buddy...after 500km and 100 candies power up the 61% guy (as long as you don't get a better one till then).

Don't power it up now, let your chances open for another use of those precious candies.

That's not too bad for level 25! They may not be very rare in your area. You should really hold off until after the upcoming event, it may give extra candy from hatches/buddy. See where you stand after that. You never know what will happen. Investing the candy you have now won't make it a competitive snorlax.

if you dont want to wait to get another snorlax power up the 84%