To start, the current complete list is for DAGGERFALL COVENANT. If Veteran Players find the best areas for the other factions, I'll add them here.
Second, many of these may NOT be the best (but we'll work collectively to make sure we get them!). I didn't re-scout Tamriel to do this, or even all of the DC area. I remembered and asked around to gather information and checked the areas to make sure they were quick and easy as can be. I know I've done that for many, but some could probably still be better.
Now that the premise has been set, let's get on to the locations!
Locations I did not include because the enemies are ubiquitous, plentiful in many areas, achieved in casual/normal/story play, etc:
Skeletons, Beasts, Insect-like Creatures, Humanoids, Bestial Daedra, Humanoid Daedra.
And now that that's out of the way too... Here we go!
Slayer of the Undead:
Zombies - North of Sentinel on beach (Alik'r Desert). Avoid the small island and you'll find your own effective path. Mine usually would circle from below the island and go as far east as north of the Scholar's Way apothecary shop icon.
Ghosts and Spirits - Elduum (Glenumbra). Easy: Go in a circle and get annoyed by the lower levels...
Vampires - Crestshade (Rivenspire). A lot of competition can be found here. The only other good spot is in The Orchard (Coldharbour), but the enemies will take twice as long to kill and there still may be competition.
Slayer of Nature:
Lizard-like Creatures - Hag Fen Wayshrine area (Glenumbra). This is an odd, fun,and yet the best route I've found. Starting at the Wyrd Tree Camp near the wayshrine, headsouth, but don't hug the coast. Hit all the crocs until you kill the one behind the rock that has a tree on it (if you got to some humanoids with a tower in the background, you've gone too far). This should be the fifth or sixth, sixth means you probably killed a backtrack one--we'll get to that. Immediately turn around and head back north, but you're going to pass by the Wyrd Tree Camp (it will be on your right). You'll pick up three (two if you accidentally killed one, which you shouldn't!). After the third, mount up and head past the said camp. On your left is a croc, keep heading left and there's another. Veer right and pass a large, broken tree with mushrooms all over it. On your right is a croc hiding behind a boulder before the tree, snag this one (explained later). Keep going straight for another croc and another to that one's left. Turn 180 degrees around and grab the lone croc in its own pool. Mount up (see, if you didn't kill the next croc, you'd have no reason to mount). Pass the dead croc, heading toward the wayshrine, and kill the two crocs and pass the Doe (it says Deer... stupid designers) and collect $200. Rinse and repeat. Should be sixteen total. Yes, even this can be done in a minute.
Plant-like Creatures - Deleyn's Mill (Glenumbra). Wide circle; lots of Spriggins here. Remember to tolerate the lower levels behavior... and zone chat.
Water-based Creatures - Waterdancer Falls (Reaper's March). I'm sure there's a better place for these things, but this is the only I've cared to use (wasn't gonna spend hours looking for another only to likely come up short). The boss counts as 3. [OR] Manor of Revelry (Coldharbour). This is the better/other method you can go. After/if you've done the quest, you can go to the water beds and farm the few Nereid around it. There's not that many, but they're much easier to kill.
Chitinous Creatures - Dreughside -> Dreugh Waters -> Dreughside route (Stormhaven). This is a rather complicated route. But you circle Dreughside, getting all of them in there, head down the hill as if you're going to kill the World Boss (that 'is' 5 or 6 toward the goal if you'd like), but looping to the left before the boss, and forming a big circle. Mudcrabs count as Chitinous Creatures so that may affect where you choose to farm, but I find this a quick route filled with tons of creatures for points. I believe it's 30+ in a couple minutes so do with that what you will.
Netches - Cave of Trophies area -> Deathspinner's Lair -> Cave of Trophies area (Coldharbour). There's really no good route for these, just look for them along this C shaped path. You could start at Everfull Flagon Wayshrine, go up to the cave, and teleport back down for a quicker route. Unsure if you'll be too fast for the cheapest fee. From Wayshrine, go north, loop around and go down, then head east and then north en route to the cave.
(Ebonheart Pact) - Outside Davon's Watch (Stonefalls). [Pending Route Information]
Slayer of Humanoids:
Goblin-kin - South of Rash Merchant's Plummet (Stors M'Kai). Circle the camp, everything has a perfect respawn timer so you'll never have any downtime. [OR[ Bonesnap Ruins (Stormhaven). On the east side of the map, there is a rectangular area with water running through it. This area has groups with mainly goblins in it, but with the occasional Durzog (may not be needed) and Ogre (which may be needed). I'd recommend staying in this area for just goblins.
Giant-kin - Shinji's Scarp (Stormhaven). There's not that many here, but you don't need that many anyway. Fastest and easiest place. [OR] Bonesnap Ruins (Stormhaven). On the west side of the map, there is a square-ish area that stands at the top of a waterfall (don't go down there!). This area has many ogres and a few goblins if your ratios better suit you here than elsewhere.
Slayer of Dwarven Constructs:
^This - Bthzark (Stros M'Kai). Easy: Go in a circle and annoy the lower levels.
Slayer of Daedra:
Flame/Storm/Frost Atronachs - Willowgrave Cavern area (Reaper's March). For those that can't solo Storm (and probably Ice) a buddy or group would probably be what you need. But, there is a caveat to that: You may not get credit if two people attack the same one. Typically, I've seen that the one who deals the most damage gets credit (sometimes, though, I've had lower levels take things from me at Dark Anchors and I was left scratching my head). This means that this achievement should be done SOLO. Lame, I know. Dark Anchors may also be an option, but the randomness isn't something I advocate.
Flesh Atronachs - Village of the Lost (Coldharbour). I would highly recommend farming Dark Anchors for not only these, but many other (vampires, zombies, other Atronachs, etc) if you want to knock out a couple more achievements at the same time. But if you're like me and realized this too soon or have been very unlucky with flesh Atronachs spawning (also like me), then this is one of your last bets. There should be 6 total in the area. 4 (3 normal, 1 boss) spawn in the lower Orc Ruins, while the other 2 spawn in the lower Khajiit Ruins. You can go in a long, annoying route between them. It's gonna suck this way, but the 200 may come quicker, but certainly come at all than other ways.
(Aldmeri Dominion) - Outside of Abamath (Malabal Tor). The outside of the ruins is littered with Flesh Atronachs--some alone; some with additional enemies. Your best bet is to find a circular route (or other pattern as I haven't took time to find a better-perceived route) to nab them all.