Dunsparce or Chikorita. Dunsparce because I've defeated a Dialga with one, and Chikorita, because that was the first Pokemon I ever saw. (Played Gold before Red/Blue (*gasp* No, it's because it came out 2 and a half months before I was born.), or watched the anime.)
I have many, MANY others (As in, the only ones I don't like are legendarys (Exceptions being Shaymin, Mew, Physical Darkrai, and Ho-oh) and some other Pokemon, (Typhlosion, Red Gyarados from the Lake of Rage, Arceus, Dark Void Darkrai/Special Darkrai, and about 15 others.) but the two above are my all time favorites.
(I only like Ho-oh because it is the legend of Gold, which is the first Pokemon game I ever played.)
(I overuse parentheses to hurt your mind, and my grammar.)