I can't recall offhand who I'VE had the easiest time raising, but mathematically, the easiest Pokemon to raise is quite likely to be Zangoose, which is part of the Erratic EXP group of Pokemon (That is, the group of Pokemon that need only 600,000 EXP to get to Level 100.) and, out of that group, has the highest base stat total of any unevolved Pokemon and it matches and even beats several evolved ones. By Level 9, Zangoose has at its disposal STAB Quick Attack, which is boosted to obscene levels by a high ATK stat and will snipe almost anything early on due to Zangoose's high speed, as well as Leer and Swords Dance to inflict even more punishment. Even a newborn's Scratch is a 60-damage attack thanks to Normal STAB.
...I'm kind of tempted to train a Zangoose now.