I decided to go with Darky's suggestion first. Why?

Because honestly Water is overdone. And I'm gonna need a hell of a lot of time to choose my favorite out of over one hundred. Dark, on the other hand, there's only about 40 of. Much easier.

I kind of at the same time have three favorite Dark types (...but in all honesty I don't dislike many), so I'm gonna put three favorites for each one I have three favorites for (more or less). xD
Favorite 1:

Spoiler:

Scrafty. I couldn't find much on dA for this one so just went with this (well, much that I liked). Scrafty is the first Dark/Fighting type in the games; which can be a double-edged sword at times. For example, Scrafty has a weakness to Fighting - and not the best Speed (but good Defense I gotta admit) which can kind of...fuck it over in a sense if you have no Dark type moves (since I'm pretty confident Scraf doesn't have, for example, a Psychic type attack...well, Zen Headbutt, but that is special - not Scrafty's...best stat.)
Design-wise, I like it. It looks like something that's...erhem, "gangsta" (last time I'll be saying that). I guess that's why I like it. xD
That should be all for Scrafty.

Favorite 2:

Krookodile.
Even if it's a Ground/Dark type (secondary Dark), it's still technically a dark type so...

On the "topic" of weaknesses, Krookodile has its fair share as well. A total of five to be exact - four of which are quite frequent types (Fighting, Ice, Grass, Water... Bug isn't too frequent I'm sure). Even if it resists four types (Ghost, Dark, Poison {again not used very often}, and Rock), it still has quite a few..weaknesses. Many of which are bad.
It's immune to Electric and Psychic, both of which I suppose are also somewhat common.
Moveset wise?
As Krookodile...
It's set to five types via level up - Dark, Ground, Dragon, Normal, and Rock. Not quite the greatest span, but it does get two very powerful moves - Earthquake (100 base power) and Outrage (120 base power). If Krookodile is set up right, it can sweep with Outrage possibly, but I'm not here to talk about competitive battling (maybe in the future ^^ /winkwink), I'm more so here to talk about my opinion of the overall Pokémon even if moves are included.
Egg moves continue this trend; just giving it Fire/Thunder fang. 65 base power each. As for TMs, it gives it 5 more types (Ghost, Fire, Grass {Grass Knot before anyone asks}, Psychic {Rest} and finally Poison {Toxic}. So all total... About 11/17 types if you use Egg-moves and TMs and not just level-ups; which I suppose is good, but I just wish it had some more...damaging type moves honestly.
/end rant about that
Design wise... It's a crocodile rock thing. What else is there to say really?

Favorite 3:

Yes, a furry magnet evolution an Eeveelution. Even if I HATE the other Eeveelutions with a burning passion because of a, erhem, 'certain image' I saw a while back (..don't ask)..
...but other than that, I'll start.

Umbreon doesn't have as many weaknesses as Krookodile. It has pretty much the same weaknesses as Scrafty if I am correct. Fighting and Bug to be exact.

Umbreon's tier I had to check Smogon for to be honest. It's Underused; which I guess could in a sense make sense. But I'm not gonna get into that part with this one because I don't know much about Umbreon competitively so...

I'm just gonna skip to design-wise.
Umbreon is based off a... Fox, maybe, I wanna say? Please correct me if I'm wrong (sam as other Eeveelutions). There isn't much else to say about it so I'll cut this short. :V

Least Favorite (only one):

Zoroark.
Remember when I said Krookodile was limited level up wise?
...yeeeeah.
Dark, Normal, Bug, Psychic. Granted, I know there probably aren't many moves they could have shoved on it, but... I don't know, I just feel they could have done a better job with its moveset.
TMs are basically the same as Krookodile's so I'm not listing them.

Zoroark is decent competitive wise, I've heard, but...
If you're using it to trick your opponent into wasting their last hit of a move you know a Pokémon you have is weak against (for example if they can only use Hydro Pump once more, and you had a fire type in the back and Zoroark left, you might switch into Zoroark to ensure they use Hydro Pump unless they suspect Zoroark being used), I guess it could be useful. However, if they ever release a DW Zoroark (...please don't.), it would lose Illusion, basically screwing itself over to be honest.

Design-wise it's just another Mewtwo, , and Lucario. Lucario used to be the popular meme - everyone would dress up as it, pretend to be a Lucario, make forums surrounding it, etc. Now Zoroark is getting the same treatment and Lucario is basically dead. I'm sure people have pretended to be a Zoroark multiple times.

/end rant about Zoroark

...that's all?
Next week I'll just do Fire I guess. Maybe grass.
- Megu