Personally I would never walk a Pokemon just for the dex entry. I would also never walk a syther, because they nest.
I love Muk, but powering him up isn't really a great idea since he isn't a relevant defender or attacker.
In my opinion, walking Chansey/Blissey is the right answer for almost anyone...
But in your case, your only Blissey is a level 4? That is way too low to power up, IMO. I would rather keep collecting the candy and hope you can hatch a Chansey or maybe get lucky in the wild.
Your 93% Snorlax is a very worthy buddy, if you don't catch them in the wild very often.
Since you apparently don't have a Larvitar, if you get one then he would be a very worthy buddy. Although once you get your first T-Tar, it is kind of anticlimactic because they are not really very good unless you can power up past Level 30.