This list was very difficult to make, but... here we are. Let's get started. Before I begin, I will refer to each Pokemon as "he" or "she" as I see them. So, bare with me.
Grass-
Maractus is such a darling Pokemon. By the way, you'll see me saying "darling" a lot. She's a very cute Pokemon, and I like the design. It's very different, as she doesn't have legs, but does have arms. I'm a big fan of cacti Pokemon, like Cacnea and Cacturne, but Maractus wins it. I especially like that she can even become her own set of maracas. One of my top six Pokemon.
Water-
Ah, Starmie... one of my personal favorites. Such a cool Pokemon. Very powerful (which I'm not usually a fan of for JUST power), and beautiful at the same time. I think it would be a great contest Pokemon, for Misty, possibly. ;)
Fire-
Not really what I'd normally expect, but yes, even I have a thing for AN Eeveelution. Flareon pretty much wins it, because it's exterior is... cute, but tough. If that makes any sense. That, and I used to prefer Fire-types when I was younger.
Bug-
Toxy, Toxy, Toxy. <3 My lil baby, she's my favorite cute Pokemon. I loved watching Jessie's Dustox in the anime, and even teared up a bit during her final episode. >: One of my top six Pokemon.
Normal-
Come on. You can't help but want to hug him. I love this thing, and it's pre-evolved form. He's definitely a childhood favorite, and a strong Poke to boot. Again, a personal favorite.
Dark-
I love cats. This thing... I love 'im. <3 Liepard is definitely interesting, and I really like the color scheme. It definitely gives the "rogue" appearance, and if you know me well enough, rogues are a-okay in my book. ;)
Psychic-
Oh, Hypno. You creepy mother frumper. I love him, and he's a top six Poke. He's definitely one of my top six for multiple reasons, one of which being from a favorite Creepypasta. That, and it's "ability" (I quoted, because it can't actually LEARN Dream Eater levelling up, but the Pokedex entries read so. >_>) to feast on dreams, like it's pre-evolved form. Again, a very personal favorite.
Ghost-
Absolutely fabulous. Sporting those gems with such finesse of a Ghost-type. One of my top six Pokemon, he's a little imp, he is. I loved using one in my Emerald playthrough. Very fun, I must say.
Poison-
The quintessential Poison-type, if I do say so myself. And, as a fan of Poison-types, Arbok holds a very special place in meh heart. From the anime, to the trading card, to the games, I always loved Arbok. One of my top six, obviously. ;P
Ground-
Claydol is a very interesting Pokemon, and I really like it. I would like to try using one in a future playthrough.
Rock-
Definitely my favorite fossil Pokemon, though I wish it had a pre-evolved form that was cute, but had the same teeth. That'd be so... amazing. <3 It's powerful, it's intimidating, what more could you ask for?
Steel-
Ah, yes. The temperamental tower of Steel that is Steelix. It definitely holds a presence of power, and I had fun using a Steelix in my previous plays. Both in game and looks, it's a winner in my book.
Electric-
Something cute and adorable? Strange, I know, right? Well, Ampharos, Flaffy and Mareep hold a very special place in my memories. I was born the year of the sheep, and it just so happens these Pokemon were introduced around the time I figured that out. Not only are they adorable, but I love using them in battles. And now that Ampharos has a Mega form, I love it that much more.
Ice-
The wall of ice, the not so nice, though it will suffice. Yes, Cloyster is definitely a favorite of mine, and I used it in a playthrough AGES ago. I like Shellder too. Such a silly face on it, but I like how it evolves into this hulking, snarky wall of ice. I love him.
Dragon-
Behold, the power of Hydreigon. I called a Dark/Dragon-type out befor eGen. V came to be, and... I got my wish in spades. Hydreigon is definitely a ferocious Pokemon, and I love her. She's definitely a Pokemon I'd want to use when I finally get a 3DS again.
Fairy-
Mawilesocuteomg. no, but seriously, Mawile NEEDED an edge in the game, and being given partial Fairy-typing AND a Mega Form? She definitely got the respect she deserves, and she's a personal favorite. Not a top six, but still. I want to use one in a future playthrough.
Fighting-
Never really got into using Fighting-types after Platinum, but this Pokemon has a certain draw I can't seem to get a grasp on. It's a good look, and it's powerful too. I'unno, maybe I'll use one.
Flying-
And lastly, my lil' Mandy. <3 Definitely a personal favorite, along with Vullaby. Such a cutie. <3 This is another Poke I want to use in a future run.