My Number One favorite. In my most recent Pokemon game, SoulSilver, Poliwag was one of the first Pokemon I acquired, and I raised it all the way up to a Level 68 Poliwrath. It even managed to acquire Pokerus, so my Poliwrath is capable of taking anyone down with its Hypnosis + Focus Punch combo.
Scorpions are one of my favorite creatures, and Drapion combines my love of scorpions and bizarre creatures to the extreme. Plus, he has a unique typing that I love using in my Poison-exclusive teams; combining his immunity to Psychic moves, his wide array of Dark and Bug-type moves, and status-afflicting moves, Drapion is an incredibly fun Pokemon to use, and an even better one to use to screw with your head.
A recent favorite. Like Drapion, Gallade fits the same role in my Fighting-exclusive teams. He's able to throw monkey-wrenches into the plans of those who try to take on my Fighting-types with only Psychic types. That's the other thing, I love Gallade's typeset, 'cause it provides for a unique array of moves he can learn. Currently in my SoulSilver game, I have a Gallade+Gardevoir Doubles team that's tackling the Battle Tower, and with Gardevoir as support while Gallade takes the offensive role, I've cleared the first six trainers in no time flat.
Muk has been a favorite since I bought Red all those years ago, but I never could use him to his full potential 'til after Generation IV. In Diamond, I raised a Muk that essentially cleans house on all occasions. I love a lot of Poison-types, but Muk has always been a classic favorite.
He's a giant sumo-wrestling (wrastlin') behemoth with hands the size of elephants. That's freaking sweet. And he uses those hands to great effect; in an online team I created, I use Hariyama as my One-Hit-KO Pokemon, combining the usual Guts + Facade routine with an incredibly high boost in Attack power, while holding the Flame Orb to activate his ability, not to mention that his other attacks hit super-hard with that Attack boost. Also, interestingly enough, everyone in my family that plays Pokemon, meaning myself, my younger sister who's obsessed with Pokemon, and my younger sister who just picked up Pokemon not too long ago, agree that Hariyama is our favorite. Guess you can call him our "digital family pet"...
Cacturne's probably one of the few Pokemon I can admit I like more because of design than I do its use in battle. But that doesn't mean I don't love adding Cacturne to my team. I've used Cacturne since Emerald, and have even transfered my Cacturne from Diamond to Battle Revolution on the Wii. My favorite thing to do with Cacturne is battle based off its ability, Sand Veil. Probably my favorite battle strategy is that of Koga's; to confuse, confound, and overwhelm with status afflictions. With Cacturne in my team, you'll never land a hit while I slowly whittle your health away. So fun.
Expect to see more of these. I've got a lot of favorites... and yes, most of 'em will consist of Poison and/or Fighting-types.