Who to walk next?
My current attacker squad comprises eight L30 wonders: DT/OR Dragonite, MS/SE Rhydon, RT/SE Golem, Count/DP Machamp, WG/HP Vaporeon, Conf/Solar Exeggutor, Bite/HP Gyarados and RS/DP Poliwrath. I have 4 Fast TMs, 2 Charged TMs, 47 Rare Candy (just spent 60 on Raikou) and 430K stardust.
Larvitar: Have walked a hatchling (13A/15D/13S) over 250km and now have 135 candy. Could already have the best attacker in the game at L30 (with enough TMs to guarantee optimal moveset) if I had not spent the Rare Candy on Raikou (but I will earn more from Entei raids).
Eevee: Obviously not a long-term solution, but mine with L28 12A/15D/14S would only need 10km, 19K stardust and perhaps some TMs to join my attacker squad. (Already have Sakura with 8A/15D/14S and worst moveset).
Machop: Rare in Singapore. Only have one because I found a huge nest at East Coast Park last March and walked one for 150km. Current candy count is 9 and Machoke is among the ~10 missing entries in my Gen 1 Living Wonder Dex.
Abra: Quite rare in Singapore (but not as rare as Machop). Currently have 33 candy and would need 17 more to power up my L23 15A/15D/14S Alakazam with Conf/SBall to L30.
Mareep: Currently have 43 candy after hatching two (one wonder). Never seen any in the wild. Ampharos is a good defender, but outclassed by Raikou as an attacker.
Togepi: Currently have 19 candy after hatching three (all wonders). Since I cannot catch them in the wild, walking and hatching are the only ways to earn enough candy for a Togetic.
Answers
If you can access Tyranitar raids you shouldn't be walking a Larvitar. If you don't have an active raid scene in your area then by all means walk em! If you get a Tyranitar under 2060 then try your luck with a pineapple. Ive had a few run but I'd caught many at 20 candies a pop - or 100 km not walked. The same goes for any available raid boss. Especially if you can solo them.
Mareep is a good choice because its difficult to stockpile those candies. Some people would call this a waste. Those people shouldn't walk mareep.
I would consider walking anything that's hard to come by with a future evolution. Especially if you already have high IV candidates lined up.
Larvitar by far. Even if you don't end up evolving that one, it's the most valuable candy you can be getting since you will want a powered up Tyranitar in the future. It's the safest investment you can make in the game besides Mewtwo. Larvitar was my buddy for 1000km.
Eevee: If you feel like you would actually use an Espeon, do that first. 10km is not much. I would not TM it though, unless you're really close to soloing Machamp but can't quite do it.
Machop: Hit raids, catch with pinaps. Much more efficient.
Abra: Not worth investing in really unless you want to solo some raids. And walking yourself another Espeon is more efficient for that purpose.
Mareep: Ampharos is strictly a dex entry. Jolteon is much better as an electric attacker and easily available. Vanity/liking the mon are the only reasons for putting any stardust into Amphy.
Togepi: You will hatch more eventually. You can also bet that Niantic will make Togepi candy more easily available come gen 4 if you're planning for the long term. I walked the last 15 candy myself but that was only because Togetic was literally the last pokemon missing from my regional dex.
