Well, this is going to be hard. Putting them order is impossible, as my favorites change so often.
Roggenrolla- I just started my first playthrough of White (Yep. Right when X and Y are here.) This ************ is the stony wrath of God. Great attacks? Check. Can tank phsical hits like a champ AND has Iron Defense at a low level? Double check. Learns Mud-Slap, my favorite move ever? Done and done. Not a fan of the designs of its evolutions, but if they are just stronger Roggenrollas (if that's even possible) then they are good in my book. I'm avoiding Bulbapedia and other such sites for the duration of my playthrough, but I'm sure it eventually learns Stealth Rock too, which I've heard is Gen V's game-changing supermove, so I have that to look forward to as well. Yeah. Roggenrola is Geodude spliced with Arceus. He also has a cute and enigmatic design to boot, and I like the derpy looking ones.
Marowak- Sick design and cool abilities. Marowak always struck me as a badass for taking a bone club to a superstrength, lightening, and tsunami fight. She's like Batman.
Porygon 2- Yeah, you read that one right. I'm a huge fan of Megaman Battle Network. The idea of a Pokemon that can enter cyberspace fascinates me. I also like 2's design better than 1, so there's that. Porygon Z is awesome too, I guess.
Slowking- I said I liked the derpy ones. I got one for my Firered version in a trade a few years back. I taught him Psychic and Thunder. It was like having Albus goddamn Dumbledore in your party! It was awesome! Also, I love his design!
Gligar- My favorite for a very long time. Ground is my second favorite type, and I just think he's awesome. No real justification, he's just a fun concept.
Ursaring- Normal is my all-time favorite Pokemon type, as it includes the most variety of Pokemon and includes badasses like this guy. Ursaring is a giant goddamn bear, he doesn't need to take your crap! He'll just smash your party and be done with it. Damn shame that you can't catch Ursaring and Gligar in the same game...