Since I was tagged to do it by kaiko_rimen here is my list of both the best and worst Pokemon in my opinion of each and every Pokemon type.
Bug
Best: Heracross
Reason: To me when I first saw Heracross when I saw it I thought "he looks really tough"
Worst: Weedle
Reason: I only found a Weedle useful when it evolved into a Beedrill

Dark
Best: Absol/Zorua
Reason: Absol- I loved the design of Absol and his mega evolution and the fact he is called a Pokemon that brings disaster although I don't agree with it entirely I think it made him sound cool. Zorua- The moment I saw Zorua I thought it was a nice little Pokemon that I wouldn't mind having as a pet
Worst: Stunky/Skuntank
Reason: They never appealed to me...at all

Dragon
Best: Noivern/Rayquaza
Reason: Noivern- One look at Noivern...I loved his design enough said for me. Rayquaza- Well...nothing can be said its Rayquaza
Worst: Goomy chain
Reason: It was confusing to me how their dragons

Electric
Best: Raichu/Jolteon/Luxray
Reason: Raichu- I'm a sucker for Pikachu and his evolved form, also in the games I had a Raichu that never let me down. Jolteon- One of my favourite Eeveelutions (though I love all of them). Luxray- One of the awesomest designs I have ever seen.
Worst: Voltorb chain
Reason: Look at kaiko_rimen  reason. For me it's the same thing

Fairy
Best: Sylveon/Diancie
Reason: Sylveon- I absolutely loved Sylveon when it was released in design and usefulness to me in the game. Diancie- It was a cool role that it had in movie and I like how crystals are used in its design.
Worst: Spritzee chain
Reason: Wasn't really my choice between it and Swirlix.

Fighting
Best: Lucario/Gallade
Reason: Lucario- A Pokemon that uses aura and is just awesome in general. Gallade- Design and to me better then Gardevoir by a mile.
Worst: Mankey chain
Reason: Didn't stand out as much to me

Fire
Best: Every starter chain/Victini
Reason: I couldn't decide which starter chain I liked better and honestly thought I like them all in someway though to play a little favourite my top 3 is Infernape, Typhlosion and Charizard. Victini- It's role in the movie...awesome, his powers...awesome, his best move...BADASS.
Worst: Heatmor
Reason: When I saw him for the first time I was like "What?"

Flying
Best: Staraptor/Shaymin
Reason: Staraptor- Best flying Pokemon to get in games. Shaymin- Loved his attitude in movie and the design was appealing to me.
Worst: Gyarados
Reason: Again check out kaiko_rimen  list, reason is the same

Ghost
Best: Giratina/Rotom/Hoopa
Reason: Giratina- Rules another dimension...what's more awesome then that. Rotom- "It's morphing time" best phrase to explain why XD. Hoopa- 1 word, Portal. 1 feat, beats legendary's for fun.

Grass
Best: Treeco chain/Chikarita chain/Celebi
Reason: Treeco chain was just really badass to me especially Grovyle. Chikarita chain especially in the anime always brought a smile on my face. Celebi- Small, adorable and travels through time.
Worst: Tangela
Reason: Weak design in my opinion

Ground
Best: Garchomp/Phanpy chain
Reason: Garchomp- One of the coolest dragons I've seen in Pokemon. Phanpy chain was both awesome and cool standing right out for me.
Worst: Gastrodon
Reason: It's design is weird for me and typing makes it annoying.

Ice
Best: Weavile/Glaceon
Reason: Weavile- His design looks pretty cool and it branches from Sneasel so yeah. Glaceon- It's an eeveelution...enough said.
Worst: Delibird
Reason: Alex Rochon on YouTube said it best in his joint top 15 worst Pokemon video.

Normal
Best: Eevee/Meloetta/Castform
Reason: All three- To me they are the best if not for the game as something I'd love to have as an animal friend or pet if they were in real life
Worst: Lopunny
Reason: Once again agreeing with kaiko_rimen

Poison
Best: Drapion
Reason: He's a scorpion...fits the role perfectly especially since in anime he's usually threatening
Worst: Nidoran
Reason: Takes up two spaces in the pokedex unlike others that changed with gender yet didn't take two spaces

Psychic
Best: Latios/Latias/Jirachi
Reason: All of them are legendary's each very awesome in their own way with Jirachi giving you 3 wishes
Worst: Gardevoir
Reason: Never appealed to me

Rock
Best:Tyrunt chain/Tyranitar
Reason: Tyrunt and Tyrantrum are T-Rex's who wouldn't love that. Tyranitar- Cool design, powerful Pokemon.

Steel
Best: Bisharp/Metagross/Scizor
Reason: It's the same for all of them...amazing designs and can be a massive powerhouse(Me: I'm looking at you Metagross)
Worst: Lucario
Reason: As a steel type I have to agree again with kaiko_rimen

Water
Best: Swampert/Samurott/Greninja/Manaphy
Reason: One word for each, starter, samurai, ninja, cute
Worst: Magikarp
Reason: Non-seriousness it's a god. But when it comes down to it one of the worst Pokemon

Well there you have it that's my list of best and worst Pokemon for each type, I can't really think of many people to tag this to (mostly because kaiko_rimen had tagged most of the people I would tag XD) plus I don't want to make it seem like they 'have' to do it sooo...I guess I'll end it with let's all continue to live in this golden age.