(in no particular order, because I can't decide. )
1.) Linoone: I don't know why, but I just love, love, love the Zigzagoon line! *squeezes* They were so useful in my Nuzlockes, and they're so cute and cool looking. :3 In fact, my Zigzagoon survived from before Petalburg all the way to the Elite 4 + Champion- and took down Phoebe almost singlehandedly. :3 Also, Pick-up is awesome in Emerald. "Hm? Rare Candies! King's Rock! Hyper Potions! Full Restores! Nuggets!" Love Linoone... :3
2.) Charizard: Who doesn't love fire breathing dragons? This was my starter in at least.... 4 runthroughs of Kanto, and I've bred so many of them in Platinum. I have a Level 100 one in Platinum... His name's Flametail, and while I'm not really sure about his IVs, EVs, or even his nature anymore (haven't played the game in years because I lost my stupid charger), he's awesome. Also, I have several other Charizards in several other games. :3 They're the best Kanto starters eva!
3.) Dragonite: It's cute, it's strong, and can totally kill anything after spammed Dragon Dance! XD The first Pokemon I ever trained to Lvl. 100... Surprisingly, it didn't take too long. I caught it as a Dratini in Platinum, and decided I was going to get it to Lvl. 55 if it killed me. :V Well, once it got there, it was finally able to earn exp by itself. :3 And beat the crap out of anything that didn't have Ice Shard. The Elite Four? Crushed by the Dragonite alone!! No other Pokemon needed! XD
4.) Swampert: I don't care what people say! Swampert is an awesome Pokemon, and it is not ugly! (also Mudkips are the cutest things ever. :3 Do u liek Mudkipz?) It has one weakness (which happens to be pretty rare in Hoenn. XD Not to mention it has Ice Punch and Ice Beam to kill those pesky things) and learns has STAB Earthquake and is just so darn cool, and Ruby has a Swampert.... (Ruby owning it makes all the difference. XD)
Also... Mudkip can do the Caramelldansen.
Mudkip Caramelldansen
5.) Milotic: So pretty! So powerful! Such a tank with Recover and Marvel Scale! :'D And Ruby has a Milotic too! And Mimi is awesome! And Milotic is just an awesome Pokemon! Totally worth fishing for four hours straight trying to catch a Feebas. :3